It is called "sub piston induction". I remember some of the older engine wizards discussing this back in the sixties in the model mags. Allegedly, it is bad for a stable idle on glow engines and most designers tried to avoid it later on.

However, I doubt that it happened accidentally in your engine, so perhaps there is an advantage of which we are not aware? I can see how the crankcase charge could be partially diluted by residual exhaust gasses. I can also see how it could lend toward relieving the vaccum in the crankcase when the piston reaches top-dead-center, thus providing more gas for the piston to use to push the fuel/air charge up toward the top of the cylinder during the down stroke.

Maybe we will get lucky and one of the engine gurus on the group will give us a clue or two.