We aren't sure.

But I'm quite sure it's not because some servers would be mining more profitable coins:
Same coins are mined, almost at the same time.

I'm going to make a test with 2 different payout address just to figure out how bad is my worst rig. Its efficiency looks pretty bad (E as defined by default in cgwatcher : accepted/getworks) . If the payout is far lower than expected considering just its hashrate it could be a clue.