"And I, being a proper Gryffindor, naturally scraped a suitable OWL in Potions and promptly dropped it." Gryffindors during the Snape era just didn't usually take to the class. It might have had something to do with his intolerance of the whole house. Or maybe it was just his hairdo.
"How Ron thought he was going to get a NEWT in Potions to become an Auror is beyond me. As far as I know Bill and Percy were good at it because they were good at everything and the rest of us blew it off."