Knockback is always in the direction in which your projectile is moving, so if you limit the number of shots fired, you reduce the expected number of times your shot will hit the target and induce further knockback, i.e. the mob will be knockbacked, but still end up statistically closer than if you full-volleyed it. Then, when you restart your initial chain-fire, you give the AI a chance to step closer to you, and therefore you increase the expected number of successful shots upon the mob's being. Don't forget that the further you knock the mob, the larger the penalty in the accuracy taken.
It's like trying to play MSs with a chain-fire weapon.