As others have pointed out, Snape was still a threat for Neville though, even after the boggart had been "Grannified" - the point is to defeat the Boggart, not conquer the underlying fear.
With Harry he fears what the Dementors bring, but he finds a way to tackle them, so if a boggart becomes what you fear the most, it makes sense that Harry's boggart experience would be different after he had defeated real Dementors (and also once Voldmort got a new body, and he witnessed Diggory's death).
Chrissy, re my post DH remark, that is by far the funniest death in the Potter books, it's so off-hand and in passing it's like