If there had been a 0 payment at 18:07, the four-payment sliding window would be 981.6594931
Gh/s Not "~850"
the average of those 4 payments (including the 0 payment) is just under .15/h.
you're right, I should have done the math myself. Instead, I just used your figure:
0.15/h comes out to 830Gh/s
There wasn't
actually a zero payment, it was a hypothetical you posited in one comment. Theoretically there could have been a payment of 0.00001 or something like that. But there wasn't.
Four period payment window means an average of the hashrate per unit of time over the last four
payments. Sometimes that's less that an hour, sometimes it's more.
The longest gap was 21 hours - between 9/19/2013 17:17 and 9/20/2013 14:17, during which time they averaged 140Ghs, and the sliding window was 188Gh/s. (And the one before that was 10 hours)
Most of the early payments weren't every hour.