Most likely his forward momentum and some nerve reaction that he kept moving.
Being a hound hunter we head shoot a lot of bears out of the tree.
Good solid brain hits mostly result in DRT the way we like them.
But strange things do happen once the bullet leaves the barrel.
Color me shocked when that head shot didn’t make him DRT. I shot a big Prince William Sound black bear one spring with a .338 Win Mag broadside through the chest. He jumped up and ran bleeding profusely. I followed the blood trail across a snow field, up a rock face, over a hill top, across another snowfield, and into the rainforest for a total of about a mile and knew that without a dog I’d never find him so you are correct - funny things happen to bullets after they leave the barrel. Plus, you rarely see hunting failures on YouTube.