Who Wins?

Lets say there is a 12 round boxing match. Boxer A is able to clearly outbox and outwork Boxer B for the first 6 rounds and then tires. Due to this Boxer A's punch output decreases to match Boxer B's punch output and Boxer A starts to beocme easier to hit. Boxer B is a much harder puncher then Boxer A and starts to land the more punishing shots throughout the final 6 rounds and manages to knock down Boxer A once in round 12. Boxer B appears to have done the damage and landed the more harder telling shots in the fight but Boxer A had clearly landed the most punches over all. Who wins? Boxer A or Boxer B?