It is indeed possible that the deceleration level achieved by the reverse thrust + aerodynamic drag is higher than the target level of the autobrake. This happens typically at the lower autobrake selections (1 or 2) combined with low weights. The autobrake logical cannot adjust this as it only can control the brakes.
Boeing has written some interesting papers which show how reverse thrust contributes to the stopping performance under different conditions. It shows for instance that on dry runways brakes are much more effective in stopping an aircraft than reverse thrust. Check myboeing.com or ask your operator.