Even people who have perfect hearing can be absorbed in what they are doing, and be startled by someone calling their name from right behind them.
I have this problem pretty regularly at work, since I work in a very open-concept office. My cube walls are only about chest-height, so people regularly approach me from behind. I also actively work to tune people out when I'm trying to focus, because there's a lot of background noise most of the time (and I'd never get anything done otherwise). So combine those two things, and I can get very startled when someone calls my name before walking into my field of vision.
In my case, I have no trouble hearing, but the sounds aren't registering as information I need to pay attention to (especially since the sound of footsteps behind me more often than not means someone is walking past my desk, rather than needing to talk to me).