In Order of the Phoenix, after
Harry, sitting in Dumbledore's office, thinks:
Harry turned his back on Dumbledore and stared determinedly out of the window. He could see the Quidditch stadium in the distance. Sirius had appeared there once, disguised as the shaggy black dog, so he could watch Harry play… he had probably come to see whether Harry was as good as James had been… Harry had never asked him…
However, in Prisoner of Azkaban when they are in the Shrieking Shack, Sirius says:
I swam as a dog back to the mainland… I journeyed north and slipped into the Hogwarts grounds as a dog. I’ve been living in the forest ever since, except when I came to watch the Quidditch, of course. You fly as well as your father did, Harry…
Sirius already told Harry that he flew as well as James, so the first quote seems like an odd thing for Harry to say. Is there a reason for this or did Rowling simply make a mistake?