Hi,  I am a newbie in mining, and I'm mining ethereum. I have 5 AMD r9 280x and according to the Ethereum Mining Calculator http://badmofo.github.io/ethereum-mining-calculator/ I should earn 0.022*5=0.11 eth per hour. But I never get much. My stats are 0.7-0.8. I use Debian Jessie. What could be the cause?