In an understated blog post on Friday, Microsoft revealed that it has sold more than 350 million licenses to Windows 7, allowing its latest desktop OS to retain the title of fastest-selling OS of all time. But the success of Windows 7 is both more and less dramatic than you might guess—a paradoxical situation that raises as many questions as answers.

"The momentum we've seen and continue to see with Windows 7 is incredible," the Microsoft blog post reads. "Customers having a positive and productive experience with Windows 7 is also an important measure. Analyst firms estimate that more than 90 percent of businesses are currently in progress with their Windows 7 migrations [as well]."

OK, fair enough. Here at Windows IT Pro, we see the same results from our readers. In fact, we left off questions about whether readers would upgrade to Windows 7 in our most recent reader's poll since previous results showed that virtually all were at some stage of upgrading. This is, I think, unprecedented in the history of Windows, but then too was the purported failure of Windows Vista. So Windows 7 is a great product, yes, but it also benefits from the fact that its predecessor was a (relative) dud.

But on to the numbers. Some bloggers—notably Todd Bishop and The Guardian's Charles Arthur—have tried to put the 350 million figure in perspective. What they found was interesting.

Vista sold about 180 million licenses by the 18 month point, a figure that is just over half of what Microsoft is seeing with Windows 7. That explains the "dud" comment above, but also the "relative" comment, as other OS vendors would no doubt sign pacts with Satan in order to have sales that high after any time period. (Remember, there are 1 billion to 1.2 billion active Windows users in the world. But Apple's vaunted Mac OS X has only 50 million users, according to Apple, despite years of regularly beating PC growth percentages. That's about 4.5 percent of all PCs.)

But what about Windows XP? Here, the estimates are a bit harder to come by, but the figure amounts to roughly 130 million licenses sold. That's almost one-third of the units Microsoft sold with Windows 7.

Of course, the PC market has changed a lot since 2001, when XP hit the streets. That is, it's gotten a lot bigger. So a better way to measure the success of these Windows versions is to compare those license sales with the overall PC sales from the same time period. But here, again, Windows 7 comes out on top, and by a landslide.

According to the figures calculated by Bishop and Arthur, Windows 7 has accounted for 67 percent of all PCs sold during its first 18 months on the market. This compares with just 44 percent for Vista and 54 percent for  XP. (Again, these figures are for the first 18 months each was in the market.)

Where Arthur loses the context, somewhat, is in his questions about why XP wasn't a higher percentage of PCs of the day. After all, XP "at the time looked like a rocket," he notes. Actually, it didn't. If you really go back and look at XP, that OS got off to a slow start. Business users were sticking with Windows 2000, which was tailored specifically to their needs and included little consumer mumbo-jumbo. Consumers, meanwhile—especially gamers—were sticking with Windows 98 (and likewise ignoring Windows Me) because of compatibility issues that dogged XP for a few years.

And then there were the security problems. XP shipped with the very worst security vulnerability in the history of major Windows releases, a UPnP bug that was found within weeks of the OS' release. This bug—which actually triggered an FBI warning—led to a series of security disasters that resulted in Microsoft halting Windows development, starting the Trustworthy Computing initiative, and retooling how it develops all software. The result of this, from XP's standpoint, was Service Pack 2. And it wasn't until SP2 was ready in 2004 that XP sales really took off. In fact, from Microsoft's perspective, SP2 was a new version of Windows, not a simple service pack.

XP's rocky start is forgotten by most people because Vista was so late to market and then was such a disaster. But the longevity of XP, and thus its apparent enduring reliability, is more tied to Vista's failures than to any inherent superiority. It simply got better over time, and had the time—more than its share, really—to get better. So we remember XP now as something special, and its first 18 months on the market (when, again, XP was so bad that Microsoft actually halted Windows development) get a complete pass. People literally forget it ever happened.

And Vista wasn't actually that bad, Apple ads and conventional wisdom notwithstanding. But in the same way that XP benefited from a history rewrite that now makes it appear better than it really was, Vista suffered the opposite fate.

OK, enough of the history lesson. The real question I have about these numbers is why the best-selling version of Windows represents just 67 percent of all PCs shipped during its first 18 months. Yes, I understand that previous versions are (or were) still being sold during this time frame. But Windows 7 is an unqualified success, and universally loved and lauded. So—67 percent? That's all?

I'd also point out that Windows 7 is the first version of Windows—ever—to sell an appreciable percentage of copies at retail. This is because it's the first version of Windows to run acceptably well on previous-generation hardware, PCs that were, to that day, running XP or Vista. But this fact also means that the percentage of Windows 7 licenses sold with new PCs is actually lower than that 67 percent figure. It's almost certainly pretty close to XP's 54 percent figure, possibly just a percentage point or two higher.

How is it that Microsoft's best-selling OS of all time goes out on such a relatively low percentage of new PCs?

My guess (and it's only a guess) is that PC makers are still selling a disproportionate number of PCs with XP. And the key to this guess is in the realization that many PC sales, especially of low-cost and low-quality machines, are occurring in second- and third-tier international markets where the cost of a Windows license represents a much higher percentage of the overall PC cost. These are markets where software piracy is sometimes rampant and accepted, markets where simply getting a PC out the door is the primary concern.

So, Windows 7 is absolutely the best-selling OS of all time. And as it passes the halfway mark between its release and the release of its successor, Windows 8, it's still absolutely a stellar product with plenty of room for growth left. But it's interesting that even such a successful product could represent such a relatively low percentage of overall PC sales. And with XP finally coming to a close from a new licenses standpoint, Windows 7 could still have its best days ahead of it.

Amazing.