The fields are not distributed in a strict level based cut-off. Back when the level cap is 90, and myself having 1 level 90 character and 1 level 89 character... The lvl 90 was in a field with lvl 85 - 90s; while the lvl 89 char was in a field with only lvl 89s and 90s.

A few weeks ago, my UK char (level 81) was sent to a field of mostly 83 - 85s, with a single lvl 89. I'm fairly certain there are a lot of players between 86 and 88, that are not in that field.

You have multiple fields that you can qualify for, whichever one you get drafted into is probably rng, from what I can tell.

Same server cluster, 90 had been 90 for over a month at the time.