Oh, and something else occurred to me too. Just after this scene, Harry tries to warn Lupin that Snape may be poisoning him; Lupin responds by drinking the potion and saying "Disgusting." He's presumably referring to the taste of the potion, but I've always thought he was also making a sidelong reference to Harry's aspersions on Snape, which would fit with the hypothesis I've just advanced concerning the use of "Severus".