Brackets are based on level, not power. So what power enemy you face is rather unrelated

Brackets are dynamic, so as more/less people join sage from a given level range, the split point gets changed. You might be bumped into a higher bracket due to fewer high level player participating, or higher level player might get bumped down into yours if more are participating.

I got to players at level 90+ when I'm only level 82

Same here

i was in the same level 92 bracket as usual, but as praiseluka said most is up on how many people joined the event and on your level. Anyways as being level 82 is pretty weird you were matched with level 90 people.