You should try another pool ( maxhash PPLNS) or maybe try solo mining luck with that hashrate i would try ETC solo mining for a week. With ethermine pool you want have the opurtunity because of to many miners to see what can cards do( to many stale shares)Solo pools are more optimised then PPLNS pools. Im only solo mining. You must make calculation on dificulty (TH!!!) your cards find on 24 h basis, but that you want find on ethermine. You should try solo mining pools (maxhash, 2miners,altpool,coinmining, clona).
This still does not explain the difference at all, how unexpected
I suppose there is maybe some pool with less stale shares.
BTW I restarted the miner and had same number effective, but after about a day, it is again about 10MH/s per rig less in effective, so seems to be caused by devfee and stales.
Maybe PhoenixMiner author could explain how these numbers really work.
After restart I even get incorrect shares, on all GPUs, interresting, before I did not get any. I have 0 memory errors on most GPUs.