Retailers usually don't have a clue when it comes to dates. They just slap something on and hope for the best.
Although I have to say, Mario Kart Wii seems like it's come around so quickly. It sounds like it's practically ready for release yet we haven't seen an awful lot of it and it seemed like they only announced it for the first time yesterday.
I guess a lot of it has to do with the fact that Brawl has been the only thing on everybody's minds, and all other games, even MK Wii, have just been not as interesting to us and therefore Nintendo haven't needed to show much of it because we're all so happy anyway.
If that makes sense XD
Point is, MK Wii has come quicker than I thought it would. Or I've just been so focused on Brawl like everyone else that I forgot it even existed.