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
It looks like there were/are several large miners waiting for payout. Last night when I checked, there was at least one miner waiting for over 30BTC of payout (including one whole block to himself), and several others in the 10+ BTC range.
Payouts are more random/variable than earnings because every user can set their own payout threshold, which mostly scales up depending on hashrate. If I were running 100TH then I wouldn't want to be paid out every 0.04 BTC. But if I were running 60GH then of course 0.04 BTC is a reasonable payout threshold.
I think most people probably set theirs for every 12-24 hours. In any case, some of the high rollers have relatively high payout thresholds (which makes perfect sense), and when those are hit then it takes a while to clear them out.