Why does Professer Binns always call students by different names?

Question

Professor Binns always calls the students with different last names - for example, for Harry, he says:

"Off you go, Perkins."

Harry Potter and the Order of the Phoenix

or to Hermione:

He stuttered to a halt. Hermione’s hand was waving in the air again.

‘Miss Grant?’

‘Please, sir, don’t legends always have a basis in fact?’

Harry Potter and the Chamber of Secrets

or to Parvati:

‘But, Professor,’ piped up Parvati Patil, ‘you’d probably have to use Dark Magic to open it –’

‘Just because a wizard doesn’t use Dark Magic, doesn’t mean he can’t, Miss Pennyfeather,’ snapped Professor Binns. ‘I repeat, if the likes of Dumbledore –’

Harry Potter and the Chamber of Secrets

Is there a reason behind this?

Answer 1

He appears to be forgetful in the extreme and has little or no patience for learning the names of his students. It's not clear if these are misrememberings (substitution of the current student's name for a former student) or whether he's just mangling them, but there certainly seems to be a pattern here; Potter becomes Perkins, Granger becomes Grant, Patil becomes Pennyfeather, Finnegan becomes O'Flagherty, etc.

JKR ascribes his forgetfulness to a simple lack of concern in learning his 'living student's' names.

The inspiration for Professor Binns was an old professor at my university, who gave every lecture with his eyes closed, rocking backwards and forwards slightly on his toes. While he was a brilliant man, who disgorged an immense amount of valuable information at every lecture, his disconnect with his students was total. Professor Binns is only dimly aware of his living students, and is astonished when they begin asking him questions."

_{^{JKR - Hogwarts Ghosts}}

Note that he gets Hermione's surname right immediately after she tells him her name, but then gets it wrong less than a minute later as his characteristic forgetfulness takes over again.