I had a quick look and
The background image width on the pages for the flip are 600px so that is likely affecting it.
If I get time this weekend I want to have a play with that code as could possibly use the same code for one of my projects, but I would definitely look at the image BG size of 600px and see what changing that and adjusting things accordingly does as you are surely getting 1200 for two x 600 so the maths says that will be something that needs addressing (though I am no expert on such things and will know more over the weekend).
Where's that page flip code from btw?