You are correct. Auditory exclusion is a documented medical phenomenon. It is a reaction to the adrenaline dump you will experience in a deadly force situation. Time perspective distortion is another reaction. Massad Ayoob documents these effects in some of his writings.Why? Well, I can only guess; but I suspect the body and brain have some sort of exclusionary auditory mechanism that can actually shut down the ears' ability to hear.
Neither am I the only person I've ever known to say this. I've heard comments like mine from other gunmen, too.