Good question there Fender, and a very simple answer. Which is that I have not imposed any restriction on the starting class - it's just the way that it's fallen. There is nothing whatsoever to stop the random number generator settling down on, say, an S Class PT Cruiser - the odds would be exactly the same as for an S Class Saleen S7.
I have to admit that I wondered how long it would be before someone picked up on this. It just shows how anything can happen when you introduce a random aspect to something. That said, I think that the fact that 3 D Class cars have come up (as it happens, the next is a D Class 2000GT, so expect Teeny-weeny Terror

) means that the chances of getting a car outside it's normal class is much reduced. Of the 6 cars so far, only the generation of C Class has even allowed a chance of a car upgraded from one class to another, and of course that ended up being the G35 which started in C.
But, this run can't last forever (can it?), so expect an upgraded car at some stage in the future.