What causes the variation in rewards between mining pools?

I've always been a miner at Brains, which used to be known as Slush Pool. I'm about to take my coins out of there soon, so I figured I’d check out some other pools. Honestly, I didn't think there’d be such a big gap in rewards, but when I tried the calculator on White bit's pool, it showed a 20% difference. Is that right? If it is, why is there such a gap? I noticed I'm getting paid more per block, but they’re finding fewer blocks. Shouldn’t it all balance out in the end and give me similar rewards? What am I overlooking?

well pools can have variance. Lets say my pool makes 1 block a month with perfectly average luck. perfectly average luck means 1 heads and 1 tails on 2 flips. white bit may have had a good hot streak and made 7 blocks in 6 months. that would mean 7/6 = 116.666% payout vs 100% with normal luck another pool could do 6 in 6 months luck is 6/6 = 100% lastly a third pool could do 5 in six months luck is 5/6 = 83.333% luck. so a shit ton of variance on smaller pools as 1 block means a huge difference. now if your pool should do 100 blocks in 6 months with perfectly average luck 99/100 100/100 101/100 1 block makes a small difference. you compared a small pool to a big pool and got a large difference it is very common

At what interval did you make comparisons? At a short interval of several days, you can notice a difference of 10-20%, but the longer the mining interval, for example, six months, the smaller the difference will be. And you need to compare pools with at least approximately equal hashrate.

Where does the mining pool get its downtime from? I've seen this at home mining pools, when the server was at the home of one of the miners. But even with such problems, the profit was higher than on regular pools.