And this is true every time?
I am almost sure, that yes.
Nice find, though I think this should be in the glitches, no?
I am almost sure, that yes. Typical programming mistake.
Now about generating a place for traps. Let's imagine, that program is generating a trap and needs a random place for that. At first, a random row and a random column are generated. If this place is proper for a trap, then this is OK. Usually random place is not OK, because appears somewhere in wall (stone). Then row will be the same, but column will increase by one (step right/east). If a new place suits for trap, then OK, if not, then column is increased again. If columns end and proper place is still not found, then row is increased and column is 1 (leftmost, westmost). If rows end, then process continues from 1st row (upmost, northmost). Unsuitable places for trap are considered walls and places with another trap, but also stairs and doors, very probably fountains and thrones. But places with monsters and/or items are suitable for trap.
The same algorithm is used, when generating random place for items and monsters. Of course, monster can step on doors and stairs, and if monster stays on door and you kill him, then it can drop a lot of items on door, but neither monsters nor items are created there.
What we can conclude from that? The westmost (leftmost) edge of room is dangerous, it consists more probably traps (and also items) as other places in room. North-south directional (vertical) caves are more dangerous than west-east directional (horizontal). North-south directional caves consist more traps, monsters and items. The north-western corner of the northern room consists almost surely something.