Three dollars and some change

Part of it, IMO, is that Harry is still not used to being able to do magic outside of Hogwarts without worrying about the Ministry of Magic threatening to cause problems for him. (Or, since he's in the US of A, the Agency or Department - or, even more likely the Agencies and Departments ^_^ - of Magic.)

The other part of it (again, IMO) is the hidden "two-tier" education system in the (British?) Wizarding World. Despite the fact that Hogwarts is supposed to be the best magical school in the Wizarding World, the classes seem to be geared towards full-bloods and half-bloods that live in the Wizarding World. It seems to be a matter of the teachers unconsciously assuming that the relatives of their students would *obviously* have already told them about *that*, so that means the teacher can spend more time teaching them *this*. Still the end result is that most muggle-born/raised students (except the Ravenclaws and other bookworms) have holes in their education.

The Apprentice