There are a lot -- I mean, relatively a lot -- of AWD models with 19" wheels out there, and few if any AWD with 18" wheels. So if you're going for an AWD, probably they're just offering you one of the ones they have available instead of waiting weeks to get a new one from the factory.
Of course, that doesn't mean it's not a return, but it doesn't stand out like a return would. Unless it's the one with a 3xxxxx VIN (most of them are 19xxxx with a couple 21xxxx but I've only seen the one 3xxxxx). Still, that might not be a return, it might be that someone's financing fell through or something.