There is no hard line between what level it separates, it depends all on population density of each level.

if there are 100 + lv 82, for example, lv 81 and lower will never see lv 83 and above. Because there would certainly be a split on 82.

On the other hand, if there is a total of only say 40 players below lv 80, then a lv 20s might see lv 70s because there is simply no lower bracket.

So leveling up by 1 lv usually doesn't necessarily move you up a bracket, thou it might. Every level up is basically a gamble, you might or might not "lose" by being moved up. Thou the thing is, if more people show up for sage in the higher level, you might be bumped down again (By the same token, you might be bumped up)

It can't have fixed brackets because:

what if there are 100 lv 40s?

What if there are only 10 lv 82 and below?

The number of active player below 80 is few, that's probably why they never have their own bracket. Doesn't mean it's impossible thou.

was the 90 and 89 in the same server cluster?

how long have the 90 been 90?