larger (proven) market share is my guess. I have no stats on that by the way, it's a pure guess. The closest thing I ahve is I looked at the humble bundle stats, and scrolled down to the first games bundle which is : http://support.humblebundle.com/customer/portal/attachments/...

Looking at the graph, I can't differenciate between mac and linux on it, but the next one (humble indie bundle 9): http://support.humblebundle.com/customer/portal/attachments/...

the mac purchases are more substantial than the linux ones. This could be due to any number of factors, but like I said, it's only a guess.