@Pierre - it was my understanding that the calibre viewer used the Adobe algorithm. And this plugin just asks the same code the calibre viewer uses for exactly the same count. Hence that is why it says what it does...
I never use ADE, nor do I have a Nook. However I just opened an ePub in ADE, and I can see exactly what you mean - the calibre viewer is reporting it as 804 pages, yet ADE shows it as 334 pages, which as you say is the more "realistic" number.
A rough character count reveals just over 800,000 characters in this particular ePub (including spaces). Which using the 1024 character count algorithm plus all the "partial" page breaks comes out about what the ebook viewer says. Even chopping spaces out only takes this down to 650,000 which is still way more pages than Adobe is reporting, so that isn't the reason.
It has me intrigued now - time for some googling to see if anyone knows what magic ADE is using...