I thought the same when I had to do it on mine years ago. Seems the adaptation will lower the reference voltage for a closed throttle if it detects that the TPS value has gone below what it is expecting, but doesn't seem to raise it. I suspect that this is because it assumes the throttle may not be fully closed for a reason.
A point that only Simon may understand and isn't really relevant to this thread but the same happens on the Leonardo on my car. After driving for hundreds of miles at a time at a fixed throttle opening, if it come off the throttle slightly, the Leo detects the drop in TPS voltage as closed throttle and closes the stepper down. If I then come off the throttle completely, it sees the lower voltage and resets the closed throttle setting so decelerates smoothly after that, It will only do it in France (or any other country where you aren't constantly slowing down for roadworks, dickheads that don't understand lane discipline, temporary speed limits and the various other things we find in this country) and can drive at a constant speed for hours at a time.