When you first sign up for an account, we are told that the "payouts" that are predicted are not set in stone. They are estimates, not pure guarantees. This is due to the fact that the price of Steem or SteemBackDollars changes -sometimes by the second. Prices rise and fall all the time. All rewards are not calculated until the exact maturity time arrives for each one that comes "due." Think of the stock market: if you sell 3 stocks at 3 separate times of the day, each one should bear a different price. Also, if you look at a stock's price on Tuesday and it's going for $x.xx you can't expect that same price to hold when you move to sell it off on the following Friday. It may be higher or lower than it was on that previous Tuesday.

Ok, now factor in your Rep, and your voting power (for curation rewards) Both of these bear some small impact on your payouts. All in all, just keep in mind that the projected "payout" numbers are only estimates -even if the actual payouts are in fact pretty darn close to them sometimes.