It's perfectly possible for developers to leave an area of reserved memory (ESPECIALLY considering the Mario Kart Channel exists) for scripts to be downloaded into. These scripts could be used to re-organise current stages, apply new rules, and so on.
For example, the 'race through ten gates' stage doesn't need to store any new assets, it only needs to know where to put the assets, which music to play, which texture sets to use, and so on. So they *could* have done it that way.
The fact that they *didn't* doesn't bother me in the slightest - I'm loving the weekly/bi-weekly new challenges and I'd certainly not enjoy them in the same way if they were all exposed at first and I could just run through them all and then get bored.
I'm at 34' 398 and I see no chance that I'll be getting it any lower than that this week