Your payout is based upon a reward pool. Let's call that P for POOL. This reward pool is FIXED in size every day.

There's no fixed daily reward pool, and that is in fact why the payouts are fluctuating. Starting with HF18, there's a reward pool balance which looks back over the last 30 days and evens out the payouts. So, a single day with a lot of activity can have a higher payout than another lull day.

Unfortunately, with significant changes in voting behavior, it has been a challenge to find an equilibrium using this system. I'm sure you'll remember how long it took for the payouts to build up over HF18. As it turns out, the reward pool balance was way overfilled right before HF19. So, for the first few days, 2x-3x the expected rewards were generated, further compounded by the fact that people went on a voting frenzy seeing their newly found influence. Things are starting to balance out now. No idea when an equilibrium will be reached, but we're getting there.

Read more here from the developer behind it - https://steemit.com/circlejerk/@aggroed/autovorotic-asphyxiation-the-mega-circle-jerk-is-choking-out-itself-through-the-rewards-pool#@vandeberg/re-aggroed-autovorotic-asphyxiation-the-mega-circle-jerk-is-choking-out-itself-through-the-rewards-pool-20170628t125153226z