My only theory right now is that they get slower as the heat up and the heatsink becomes fully soaked. Not sure about that, though.
I had the same thought when I saw the hashrate increase after the slight undervolt at 550MHz. The avg hashrate is at 2.12Th/s by now, I think I will just leave it running like this for a few days to see if it goes up even further yet.
That's impressively high for just 550MHz. After a few hours of running 700MHz, mine seems to have settled on around 2.3Th/s, after starting at ~2.5Th/s... Still not sure if this is temp related.
my 2 r909 miners have been running for ten days now and the performance at 680 and 640mhz is impressive
both pod miners have settled between 2.34 and 2.45 th/s
4: GSF 10070009: BM1397:06+ 680.00MHz T:680 P:662 (3:2) | 90.1% WU: 90% | 2.838T / 2.457Th/s WU:34332.8/m
5: GSF 10070003: BM1397:06+ 640.00MHz T:640 P:630 (3:2) | 91.0% WU: 91% | 2.128T / 2.343Th/s WU:32744.4/m