And I suspect it has to do with the number of pages in the sewn signature of a bound book: 25 would be to signatures would be two with forty-nine printed pages and that blank page that seems always to be there. Or 16 (the number Dover seems to use, at least for certain books) would be 3 is 48 plus a mystery sheet. If the definition comes from the ISO, it is almost certainly some such reason. (See their explanation of the A size papers).