Or perhaps you have a follow through that involves a "wave" ie the gun is held on for as long as you can, but involves a slight sway of the gun the same each time. slightly slower ignition/firing can lead to a consistant POI that is "off" the same each time. If you watch some shooters in offhand, when they are aiming, the muzzle actually makes a slight circular motion. if they try to shoot as the gun sways this way, it can be off a bit. I once belonged to a club near Annapolis that had a range down in a swamp. There was muck and water between the firing line and the 25 yd backstop. Summers were so humid there that we had to constantly wipe the greasy runny fouling residue out of the pan and barrel.