There was a thread similar to this, someone suggested that they're both EXP & Power based, but the field only has a certain amount of slots on top of there only being a certain amount of fields, apparently there's low level fields & high level fields, it fills both fields up with as much accuracy as it can & if there isn't enough spots in one field it'll place you in the other.

Taken for granted what they said it should work this way for the cross server version:

if there are lets say 400 players that register it creates 5 fields, 4 filled with 75 people and one with 50. Lets say 100 are level 95, 50 level 94, 50 level 93 and the other 200 under level 93. Those 5 fields own 3 factions so are 15 factions in total.

The 75 strongest level 95 fill the 1st field and are distributed in a way that 1st, 2nd and 3rd are in 3 different factions. 4Th is with 3rd, 5th with 2nd, 6th with 1st, 7th with 6th, 8th with 5th and so on up to the 75th.

The 25 lowest power level 95 go in field two and are distributed there as if they were the top 25 powers with the strongest 94 that is actually considered like if was the 26th in the list, with the 2nd strongest 94 as the 27th and so on until the field is filled. If the numbers are the one i said theoretically should not be there any level 95 or 94 in the 3rd field, and the distribution goes on in the same way.

But the truth is that DOESN'T WORK AT ALL THIS WAY, because actually seems there are some seeds and also some random people that doesn't go where they should belong to.