What are the odds of getting dream jobs for certain levels?
My observations are that the dream job distribution appears random across possible levels. With many bitizens coming through, there are bound to be some repeats from time to time and they may appear to be clumping into groups. It's easy to see patterns in randomly distributed data that aren't really there and I think that's what's going on here.
The dream job distribution is random, and so from a probability standpoint you will get several bitizens with the same dream job, but none with a dream job of another level.
Most users choose to evict bitizens after they have maxed a level with "dream jobbers" in order to solve this problem.