There's two reasons though - they're not hard boundaries, but probabilities, and they've changed over time.
There's some people who have used level editors and done graphs of actual distribution. Might be interesting if anyone has done that for each release to see how things have changed.
Hmm, I might be thinking of the old server, or of someone else's hole. Something has a big long ladder all the way up, and a section where the whole wall is covered by ladders.