Yep, like Hashnest etc, the daily maintenance fee can affect payouts if Bitcoin falls in price against USD, EURO etc. So when GM do the payouts they are taking out the fee at the latest BTC/USD exchange rate so if Bitcoin has fallen in value then it affects the amount that is taken for maintenance.

I believe Bitcoin fell under 200 USD earlier today..is hanging around 232 USD on Bitstamp as i write this.