Like everyone else, I always thought that Boss products were very good, until I discovered that their expression pedals are not. I have opened them up to see if they can be adjusted, but it is not possible. It is just poor engineering. You would expect better from Boss, but it is what it is. My work around is to fix a pad under the heel end of the pedal, so that the travel stops when the calibration reaches zero. You get less pedal travel, but all of it is consistent.
Two previous threads cover this issue, and confirm that other users experience the same problem.