If I remember correctly, deceleration "generally" maps to FTC 21. Here is my opinion (your original post sparked me to study this):
The values for FTC 20, 21, and 22 are learned between the time when car first goes into open loop and the evap begins to purge (or is done purging, I can't remember off the top of my head -- but regardless it happens shortly after starting the car). Once they are learned, it locks for the duration. Thus, the VE cells that the car is in during your deceleration are NOT the sames ones that FTC 21 was learned in. I would look in your log to see what VE cells they are actually learning in and change those to affect your decel issue.