Hi,
can you tell me in the statistics menu what the difference is between printed and paperback pages? Thank you. 
One calculates for standard paperback sized book (this is an estimation, of course, and can be tweaked in the Options tab), and the other for printing from your computer, and it takes that from your current print settings, so probably either A4 or Letter, depending on your default.
Paperback setting considers six characters as a word and uses the number-of-words-per-page as set in the options pane - as AmberV says, it just gives a rough estimate of the number of pages your draft might have were it printed in paperback form.
Best,
Keith