ToME: the Tales of Maj'Eyal

It seems to be completely random to me, but everytime I visit specific zones, I seem to start in the latest (final, boss) level and I have climb my way up to the easier levels and exit. Is there some rule for that? Where is the rationale for this? Example zones where this was encountered: Rhaloren C...

I don't understand under which rules the game decides to put me into original or alternate zone layout, which offers different bosses and in some cases different surroundings. Is there a rule? As it seems completely random to me and there is no clue anywhere in the wiki and/or forums. I'm playing th...