So you want to say that, the higher the level you are, the rarer cars you can encounter? That's the first time I've heard of this theory.
http://gt5cars.net/
This website predicts the UCD cars, and I tried it once and it had predicted the right cars, so it works. But it doesn't say anything about A-spec level whatsoever.

It's all about a game day cycle I've heard many times.