Well, just like humans, I think that dogs just have their own different personalities, some are barkers and some just aren't. Years ago me and my sister adopted two German Shepard puppies, brothers, and as they grew up mine wouldn't bark at all unless I said 'Speak' Because I taught him to do it when he was little. Mine learned all sorts of things and my sisters.. Even though I trained them, he just wasn't too into being trained.
As long as your grandma is getting the pup checked out, the vet will typically notice if anything is wrong and if your grandmother has experience with them, it's likely she knows what she's doing to some extent. I'm sure the puppy is fine.
