Because he´s not acting like Peter Maximoff, he was sarcastic but also playful and liked to help people and be there for his family, from what little we have seen of this Pietro, his sarcasm is more hurtful, not playful and he spoilers:end of spoilers
knows Vision already died twice, so he´s way more aware of this world and who wanda and vision really are than Monica or the rest of the neigbours, he also could be a possesed Pietro, that would make sense.