I find it hard to believe. Why would the PC version need 3GB when the 360 version can cope with less than a third of that!?
Because the Xbox360 is unalterable, dedicated hardware. It's much easier to make a game for that. Console specs are completely different to PC ones, a console will punch well above the equivalent on a PC.
The draw distance in Assassins Creed is excellent too, so it wouldn't surprise me. It's recommended, not Minimum.