See frnds, the 6th book seems to be quite thinner than the 5th one:
Update: HBP confirmed at 672 pages
After checking with our sources, we can now confirm that the 672 page count was given to both Amazon.com and Barnes & Noble by the US publisher Scholastic.
We've also been in contact with Bloomsbury, but they're unable to provide us with a number of pages for the UK edition at this time. It is typically less because they use a different font.
thnx to mugglenet for the info.