I also can't figure out where those numbers come from.

I'm guessing a part of the confusion could be that there was also a rerelease called SimCity Classic. The Classic release also came as a Windows 3.x version which (unsurprisingly) had higher system requirements. The specified minimum for that was a 286, though, not a 386.

I have no idea how well a 286 would have coped, though; it's rare to see "real" games (bigger ones, not Solitaire) for Windows 3.x that didn't require a 386.