Not that it matters much, but I think it's due to the way pages are printed and bundled together into sections (graphs?). Many printing presses are large enough to print 4 or more pages at a time on one sheet. Then the sheets are cut and folded into sections. This means the number of pages printed and pages per sectio have to be a multiple of 4. So one often ends up with a few blank pages at the end of each section. Nothing that can be done about it without some heavy-duty computer page composition, easier now, but not even remotely possible before 1970 or so.