I also thought this first. But in my case, hashrate with multiple instances varies a lot (from 820 up to 940). Rejections gather around 3.5%.
My system hangs some seconds between the hashrate-calculations. I think, the "plus" of hashing power comes from that time intervals get inaccurate if it comes to such a hang.
When I mine with only one instance per GPU, there are no hangs and hashrate is about 774 - 775 MHash/s. Rejections are about 1.4%. I think this is more accurate.

By the way, updating to 883 worked without problems. It also restarted itself. However, I had the strange effect that the control screen was r838 and the mining screen was stil 870 . Had to kill smartcoin (option 2) and restart it. Then both were 838.