I also noticed the same, but I believe it's due to luck and timing.. as in a good string of luck will accelerate payment, a bad string of luck will shelve shares and delay the next payment.
Is that it? I've noticed over the last month or so that although variance on the pool is low, payout variance seems to be all over the place. Granted I'm only a 220gh setup here, but I have seen the payouts being delayed.
Perhaps it's the impact of very large miners?
C