It's the difference in voice.
With dogs sensitive hearing, I can imagine that a dog thinks that high pitched voices come from small things,
therefore it's something to play with. Deep voices come from big things, which dogs find intimidating.
I have a friend, who has a dog, who is very playful and gets over excited when visitors come to her house.
Being the only dog in the household, naturally it wants to play and jump all over everyone in excitement.
Regardless, it's a good dog.
I watch my friend try to calm the dog down for a few minutes, using a high pitched woman's voice, saying:
"That's enough, settle down now" but to no actual change in the dogs behaviour. I let her try for a few more
minutes before I step in with a deep, stern voice saying: "Settle down you mongrel dog!" and the dog does
instantly settle down.
My friend thinks I am mean to the dog, but fails to realise that I don't use the same deep aggressive tone
when I say: "Who's a good dog, dog want a bellyrub?" If I was mean, her dog wouldn't be curled up near
my feet hoping to get belly rubs. Which the dog gets anyway, as a reward for being a good dog.