My dog can hear again?

Is appears that quite out of the clear blue sky my dog can hear what I'm saying again. He is a 15 yr old large (110#) mix breed that I verified to be (just as suddenly) deaf a year ago. We went as far as to sneak up behind him and ring a bell behind his head to check for reaction- deaf as a post. We have been getting along just fine with the hearing loss with body language and such, but just this afternoon I noticed his ears pivoting, I haven't seen that in a year at least. So then I tested him out by slamming doors (you could attribute to vibration but...) and calling his name from various places not in front of him. He is even obeying commands like lay down and get up. I even got him to sit and I haven't figured out a way to tell him to do that since the hearing loss. Don't get me wrong, I'm thrilled but I am just dumbfounded by the reverse in this situation. No, it wasn't that his ears were dirty or anything like that. He has been checked by his doc on numerous occasions because of a plethora of others issues that accompany providing end of life care for a geriatric pup. I'm curious how hearing can spontaneously return after (seemingly) total hearing loss?