You should only see that message if you choose the "Estimate from content" option and you don't have an epub or mobi format. Or if you choose the "Download from Goodreads" option with a word count column (and no epub/mobi). If there is some other combination that you are seeing it on, that is a bug that should be fixed, so if you see it again let me know and the details of what formats and which option you chose.
I'll take a look sometime at the mobi page count algorithm stuff, maybe next weekend. I've become a word-count fan myself, took some getting used to the different scale but it offers a better consistent indication of book size. I still use page count though, just so if I take a peek trying to figure out what to read next on my Kindle I have some idea of how big it is
