It's probably
- player level/attribute points/skill points/WPF points spent
- equipped item value
- kill/death - ratio
- the class of each player, if being infantry, ranged or cavalry
- the generation of each player
and this is being used to calculate the teams as equal as possible.
And after this is done, banner balancing rounds out the balance. That's where the completely fair matches and missing rape rounds origin from.
I think this might be too complex for me to believe it, but perhaps some of the values are measured.
For sure it is the result of the autobalancers calculations. I bet it is 3 values:
Player levels
Players historical K:D ratio
Players Kills for the round
IDK how these are weighted in the formula however. But the autobalancer tries to even out the teams, and it has a tendency to start shifting the heaviest (in terms of the above numbers) players first. That's why top players on a team more often get's shifted to the other team.
Could be wrong of course.