There are some things that you learn in school, but not in the standard curriculum that students are supposed to learn.
Hermione Granger guessed Harry Potter learned by influence, learned by being under Professor McGonagall's care for six years.
She guessed that Harry felt that being in McGonagall's House and catching these things were inevitable.
Possibly, they have been with McGonagall for so long that he has begun to show signs of influence because of her.
"Maybe you have something in that bloody beaded bag of yours." He said.
Hermione's head ticked.
Bloody beaded bag…
Where did she hear something like that before?
"I will not have you, in one night, besmirching that name by behaving like a babbling, bumbling band of baboons!"
The day before the Yule Ball!
He's speaking like her.
Of course, whenever the three would get in trouble they would always face her, add to the Transfiguration lessons and being members of her House of Gryffindor, and her help for the Triwizard Tournament, it would be difficult not catching the way she speaks. It was inevitable for most men in Hogwarts.
McGonagall was powerful, she knew how to stick things into people's brains and successfully turn them into habits.
Harry Potter, despite all the scrapes his wretched scar has brought him and his friends, was really a good student.
I mean, who would listen to McGonagall speak like that AND catch it like the common cold?
Only that this 'speaking-in-the-same-consonant' manner could not be cured with a swig of Pepper-Up Potion.
Harry Potter, no matter how much he tries to rebel and skip classes for the 'greater good', was still a student.
Harry Potter was Minerva McGonagall's student.