Inconsistency (nerd alert!)

I am only writing this because I know that most of you my readers have read Harry Potter and are eagerly anticipating the release of Harry Potter and the Half-Blood Prince. I'm re-reading the book in preparation and noticed an inconsistency that I wonder whether any of you can reconcile. I'll give you a moment to retrieve your copy.
Dumbledore takes Harry into the memory of Bob Ogden in the chapter "The House of Gaunt". In it Morfin is speaking parseltongue, which Harry understands but Ogden doesn't. If it is Ogden's memory then how does the memory include the Parseltongue recreated accurately enough that Harry can understand it?
It's just been bugging me today. Nerd moment managed. 10 days to showtime!