Services Packs, from SP4 onward, offer the option to create an uninstall folder.
If you apply a Service Pack, and subsequently reapply it, choosing the option to create an uninstall folder, you have just over-written your previous unistall folder with the files from the current service pack.
In this situation, an uninstall simply puts the current service pack files back.
To guard against this happening to you, copy the uninstall folder to a backup folder. If you ever accidentally over-write it, just delete the contents of the uninstall folder and copy your backup files to it.
NOTE: Microsoft does NOT support copying your previous Service Pack files to the uninstall folder.