It was up against three books with new movie releases and big advertising campaigns behind them ("If I Stay" and "The Fault in Our Stars", and "The Giver"), a book that has a massive hit TV show premiering right now ("Outlander"), two Rick Riorden books (guy who writes Percy Jackson books, also movies), a new Kevin Follet book (he's a huge bestseller, and his books are made into big TV shows), and a new Jack Reacher book (made into movies), and Divergent (a hit movie, current big on DVD rentals). And all that is ignoring the elephant in the room - it was back to school week, and every friggen back to school book was spiking at that time.
I would say the 5e PHB beat out massive competition for that spot.
Anecdotal evidence from selling books (totally separate from any B&N official statistics), from what I've seen with the popular books and the school books, I'd agree with [MENTION=2525]Mistwell[/MENTION].