There are a few reasons. The most common is just voting power dilution. We have a certain amount of voting power that depletes as we cast votes. The more we vote, the less our vote is worth. The power replenishes daily but this is why it's perfectly normal to see your payout projection drop... unless you're a an arrogant douchebag that pisses some of the larger stakeholders off and gets flagged down to zero in minutes. ;-)