It'll be the way Scholastic publishes their books, I guess.

The actual page count of the US version is 784. Making it the second longest behind Book 5's 840.

I have to say, I prefer the US cover art (http://media.movieweb.com/news/03.2007/hp1.jpg) to the UK one (http://www.comingsoon.net/nextraimages/hp7ukchild.jpg).