I certainly hope it's an averaging scheme, where all workers are getting paid for their portion of work completed irrelevant of what task it was on... although that might complicate matters regarding minimum profitability settings.
I'm pretty sure it's not.
15 minutes ago I was mining @ 5.5BTC/GH, now I've been switched to 5.9BTC/GH, even though there is two 6.5BTC/GH orders.
I am actually pretty sure it is an average. The chart shows my profitability has been fractional amounts like 5.54, 5.88, 6.12, etc since the change, and fluctuating much more rapidly. Previously it was 5.4 for a while, and then 5.6, back to 5.4, etc.
Last 5 minutes have been at a profitability of 5.75BTC/GH/day.
A very rough average based on GH limit rather then real GH rate would be (6.5*.1)+(5.9*.3)+(5.8*.2)+(5.6*.4)+(5.4*.01) = 5.874. Which is close enough to make me think the payout rate is definitely an average.