I came to a sudden realization…
"And what the ruddy hell are dementors?"
"They guard the wizard prison, Azkaban," said Aunt Petunia.
Two seconds’ ringing silence followed these words and then Aunt Petunia clapped her hand over her mouth as though she had let slip a disgusting swear word. Uncle Vernon was goggling at her. Harry’s brain reeled. Mrs. Figg was one thing - but Aunt Petunia?
"How’d you know that? he asked her, astonished.
Aunt Petunia looked quite appalled with herself. She glanced at Uncle Vernon in fearful apology, then lowered her hand slightly to reveal her horsey teeth.
"I heard - that awful boy - telling her about them - years ago,” she said jerkily.
"If you mean my mum and dad, why don’t you just use their names?" said Harry loudly, but Aunt Petunia ignored him. She seemed horribly flustered.
I definitely did not notice this the first, nor the second, time reading it. Jo found yet ANOTHER point to show her amazing creativity and foreshadowing. “that awful boy” was not James, but Snape. But to top off the fact that we wouldn’t know it, she pretty much asks the READER not just Petunia why she didn’t call them by name. Because we didn’t know yet; we weren’t meant to know yet; Harry wasn’t meant to know yet. It was all in Jo’s plan♥