Apart from the pool fee and the coded exchange rates, there is one more way to skim some miner funds. I'm mining something with 2 GH/s, I get 1.7-1.9 on the website no matter if I leave the varrdiff or set the local diff with the password parameter (and it's overridden sometimes). Is to be expected? Not really, not in the long term, but ahash is so obvious about it, that you can put the same miners in hashrefinery and you will see the difference. Also I've no idea if this is only a frontend bug of yiimp, but when you send 30 shares a sec and you get one rejected, you will still get some insane percentage of "rejected". You get some percentage even if it is rare as a hummingbird on the North pole. As long as you have one rejected share, you get at least 1%...

You are not making accusations, but you should, as this is greedy.