It depends on how 'lucky' a pool is to find blocks. If a pool finds less blocks today, your daily payout will obviously decrease because there is less that it can pay you.

I'm not sure how they calculate the maintenance fees and how fair it all is, but the rate at which blocks are found is crucial and can't be measured by calculators perfectly. In that regard, always keep in mind that payouts can be higher or lower with so many variables playing a role of importance.