The monster level tries to summarize an enormous number of variables that impact just how dangerous a particular critter is. Clearly, that's not going to be an exact science. Take a few of the critters listed:various alligators/crocs: good hit points, high-damage attack...but melee only, no special attacks, and very poor resistances. They're really not that hard to take out, so they get a lower monster level.
psi sharks: nasty, but one spell basically neutralizes them. If you don't *have* that spell, you have problems. (I didn't have it, the first time I went through that area. Had to use the terrain to isolate them, so I could take them out one at a time. Still consider that fight one of the better-executed fights I've run in the game...couldn't just overpower them.) The psi shark/death ray combo...there were 10 of em. You can *expect* to have problems.
They're also using more diverse attack types, and all can attack from range.
death lords: nasty buggers. Very tough if you have to move to get to them; not nearly so bad if you're on top of em.
Fundamentally, it's a matter of recognizing what the monster does well, and denying that. As the monster level increases, that does tell you that either a) it does several things well, or b) the one thing it does well, it does VERY well.