Why is my dog acting like this?

You said it yourself. She doesn't like strangers, which your grandpa is as far as the dog is concerned, and she doesn't like men. A strange man standing up or moving around is more of a threat, in the dog's eyes, than a strange man sitting down. It doesn't matter much exactly how the dog got to be that way. What matters is what you do about it. What do you do when she growls and barks at him? If you don't correct that behaviour immediately, you are telling the dog that it's okay to do that and she will keep doing it.

She wasn't socialized enough to accept people in general. & she was never taught not to bite.

She should have learned both as a very young pup of 8 weeks old.

Why are you allowing this poor behavior and not correcting it immediately? Apparently he was never properly socialized beginning the moment you brought it home.

I feel like your dog is possibly just nervous or feels threatened by males in her presence. For now keep her away from males to minimize injuries on both parts and consult your vet or an animal behavioralist.