Different hashrate in another pool

[skynet6542]

Hello,

i'm new in mining, but i have only one think, which i don't understand.
I'm trying to mine ETH with ethminer  on two different pools with same miner, same hardware, same overclock and same settings. But I have on minergate hashrate about 51MH/s and on ethermine abou 35MH/s. Can you tell me, what makes this big difference between this pools?

Thanks