I never paid close attention to this before. I'm sure the backend guys can help explain it from their perspective as well, but I'm gonna throw it out there that the increase in the actual payout in comparison to the preview is because of the curators' HP stake fluctuating, with a general trend of increasing upward.
Or, simply, and I'm willing to bet on this, it could be that the HBD part of the payout isn't taken into account when displaying the preview. So that knocks the difference down to %100, which makes sense.
apparently the payouts are in $ and should be indicated as such by the frontends