Another sfc /scannow question

This is kind of 'building' on this thread: http://forums.majorgeeks.com/showthread.php?t=165928

I'm now running SFC on a PC with WinXP Home SP2. When SFC prompted for the CD, I popped in a regular XP Home SP2 CD (not branded from Dell or anything) and I got another error saying that it was not the correct CD. So, I grabbed an XP Home Upgrade CD (one that I basically created myself by slipstreaming SP2 into an old pre-SP1 install disc) and SFC ate it up and continued from that point without any more prompts. I checked the CD drive and it was reading from the disc, so I know it was working. How did SFC know which version of XP I had? Why did it reject the XP Home full version, but it liked the 'home made' upgrade CD? And where in the registry would I find this setting, if such a thing exists?

Thanks!