I think it's relatively standard practice to add/subtract clicks when you find the POI has changed (from shoot to shoot)
So to answer the question "why" It's certainly not the scope moving about! But could be a number of other things.
If damp air is slowing down the pellet more than normal, then it stands to reason that some adjustment needs to be made.
If the gun is producing the change in POI (as apposed to the atmospherics) then my old method of dialling out the difference and leaving it would have been ok.
But if it's atmospherics changing the POI, then I would expect the shape of the trajectory to be different, thus needing a more complicated solution. ie more clicks for longer ranges.
I hope I'm making sense here!?
Anschutz 2013 MEC1 .22LR
Miroku 3800 12bore