Well, actually this might be a mystery, Windows mostly fails to unload any files being in use so it creates a second version and applies this after its reboot.
Personally I found out that after searching for the physical update process .exe file (wuauclt.exe or something). There were about 3 versions of it left behind (this was on XP though).
My suspicion is that it never throws away these old things because Windows makes a nice restore point (that you should want to keep), and that restore point obviously needs the old files to work again.
You could go out and find every thing the computer updated, but that would require digging trough piles of update history, system logs and microsoft knowledge bases.
Personally I wouldnt go hunt for these 2 gigs of updates, when windows updates succesfully, and it has actually fixed it, do not mess with anything it has done. You might just upset it.
And with hard drives costing less then a dollar per gigabyte, you probably get paid more per hour then you can retrieve per hour
I'm not a Microsoft specialist, I'm just assuming this based on my experience and logic.