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.