Super minor quality issue, I assure you.
I just remember seeing this posted by x4000:
For practical purposes, you also must not put too much room at the top of a stairsdown or at the bottom of a stairsup. Essentially, there must be exactly 3 tiles on the top of stairsdown, and at the bottom of stairsup. This way only the one door will seed there -- the door from the more-central floor -- and then all the other doors (which are all on the more-distant floor) will spawn at the far end of your staircase/ladder/whatever.
And so when I saw this stairs down:
http://i.imgur.com/4SuUj.jpgit stuck out at me.
How do I determine what map that is, and get it, if not fixed, at least adjusted a bit?