Probably the hammer was pushing the tophat into the bottle and it was bouncing shut before the pellet got pushed out of the barrel? When you adjusted back out, it kept the valve open just the right amount of time.
Well don’t know if this will help much, but before I bought my gun (SS)from the seller, I had it cronied on 2 different PW settings. On the fullest power, it did a 872 with 14.3 gr CR wad. I took it down to power 4, and with the same gr pellet, did a even 600. I usually shoot at PW#2, for backyard plinking, don’t know what the fps is, but it feels consistant shot to shot. My gun is also older, the seller bought it in ’05, and has been sitting in a rifle bag ever since till I bought it 3-4 months ago.
Mac1 Tim told me the PW on the older guns actually have a better spread than the newer ones, although I’m not sure why this would be, he didn’t elaborate on it.