I have always mined at Brains (formerly Slush Pool).

I am getting ready to withdraw my coin from it soon, and decided to shop around the pools. I never really thought there was going to be a drastic difference from one to the next, but I used the calculator on White bit's pool and it's showing a 20% difference in my rewards.

Is this accurate? If so why such a difference?

I realized I'm getting more per block, but they find less blocks. Shouldn't it even put in the end to be about the same amount of rewards?
What am I missing?