Just checked, and they are pretty even, no idea other than crazy good luck for Anon.
I did run my rigs on n50 and I did find a number of blocks and got the expected payouts I had worked out I should get last week, but the RTD from Australia was killing my effeciency, so have stopped mining cash at the moment.
Wish mine big would sort themselves out.
But the good luck is pretty consistent. Before moving to N50, I mined a while on Minebig. My workers generated 9552 Cash but I only got paid about 800. I suspect there are some bugs in the reward calculation:
http://i.imgur.com/gUwGtwz.jpg