It's a bit less expensive because you get ZERO support and the version that you buy is THE ONLY version on the disk. Don't ever buy OEM, I got screwed the last time I did that (XP Home SP3, had an epic fail, went to get support from Microsoft and it said "Your version of Windows appears to be an OEM version. Microsoft does not provide support for OEM versions of Windows. Please contact your OEM to receive support."
"Don't ever buy OEM", what?
There no reason to ever buy anything except the OEM version of Windows. Microsoft support is useless anyway. Search for solutions to your problems yourself, that's what Google is for. Even if you plan on upgrading hardware you can call MS to get your key working again should it start bitching.