Reverser is not taken into account (regulatory requirement) on dry runways.
The delay after touchdown before the autobrake LO comes into operation and the effect of the reverser in the meantime should explain the difference.
Also. you need to apply, I think, DRY/WET check, which tells you to disregard any wet runway results if they come to a better figure than those calculated from dry.
As far as I remember airframe drag (from spray) only enters the equation on contaminated runways.