Because Vista virtualises file I/O and even some explorer views - sometimes what you see is not where you think it is.
If that was the case, I shouldn't be able to see that high score file in all my accounts. I ran my game in the standard user mode, achieved a score and exited program. Checked file, score there. Switch to my admin account, run the game again, and the previous high score is visible. Shouldn't it have created a new high score table because I'm now in a different account?
If I remember correctly from the doc on MS's site that was linked to on another thread (anyone got that link?), if virtualisation is being used I should not be able to see a file that was created in a different account to the one being used. This was one thing to look out for to see if virtualisation was being used by a program.
EDIT - Link to MS document referred to: http://msdn2.microsoft.com/en-us/library/bb206295.aspx