Must be because of what is known in paintball and airsoft circles as "barrel suck" - a partial vacuum forms behind the ball or BB, slowing it down (in this case, even stopping it). There's not enough air in the piston cylinder to completely displace into the barrel bore to push the BB out. Even as the BB already has momentum to keep moving forward.. in itself starts acting like a piston being drawn out in reverse, especially if its a tight fit inside the bore. The BB may literally stop where the head of the air column stops moving thru the barrel. The bleed hole is not intended to vent pressure out.. but to let air in to equalize the negative pressure. With "barrel suck" eliminated, the BB can keep moving on its own momentum.