network diff. is constantly changing so when network diff. is high valid shares are very diff. to find, earnings decrease and that is the variance in earnings 
https://explorer.zcha.in monitor this site and see for your self
https://explorer.zcha.in monitor this site and see for your self When pool mining, your share diff should be constant no matter whats the network diff. (It may change a bit, but thats not related to network diff)
You will see changes on vardiff, cause logic on pool side keeps adjusting to your sharerate.