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.
And by doing so, you contribute to the problem even more, making it more difficult for the smaller pools to get an equal share and centralizing power even more until it won't be possible to reliably mine anywhere else due to the large luck-swings...
The reason why you get less is because of the variance. On a smaller pool you need to mine for longer to average out the same amount of blocks as you do on bigger pools.