Historically, one of Peter's major weaknesses has always been his restraint. He isn't always running at his utmost potential, especially against somebody like Castle, who at the end of the day is an ordinary guy twice his age with a pistol. He's almost always holding back, especially against somebody who really doesn't stand a chance on paper.

When Peter shows back up towards the end of the issue, berserk with grief, he comes within an inch of killing Castle and only loses because Peter's gone insane. Castle gets his attention by name-dropping Aunt May, and in that moment of hesitation, Castle puts a few rounds into him.

It's a Kurt Busiek comic. On the page, he does everything short of flow charts to justify the situation, and there's exposition that these panels don't show. It's not really worth trying to nit-pick it based on power levels.