I mined for a week on miningspeed a month ago and got about 25% less coins than WhatToMine predicted. I then Switched my rigs to Suprnova and got the right amount there.
I'm all for spreading the hashrate but I'm not for less reward.
I am guessing that diff went up constantly in a time when you were on a first pool. I know it was a reason in my case, I was mining from below 1Mhs and I saw how the reward was dropping. We are at about constant nethash for some time now.