Dublinpilot
I use a rough rule of thumb that says -50rpm = 100fpm descent on a fixed prop.
So your theory may be spot on - the pilot would unconsciously hold back pressure (or retrim) to prevent a 200fpm RODwith the carby heat on and the A/C would slow.
On returning the heat to normal, the AC would need to accelerate back to the trimmed speed or be retrimmed and a drop of 40rpm doesn't sounds daft at all.
Anyone else like to comment?