Hi!
I see the analysis considered only (English) Wikipedia, but I think it is important to keep in mind that the kind of material (articles) produced on Wikipedia is quite different from what is created on Wikibooks and Wikisources (mainly Books). There are discrepancy also in the kind of readers and editors of these projects.
Considering that the first version of the extension focused on providing Wikipedias some way to create "books" (simple collection of isolated articles), the result was a tool which is not very integrated with the wikis where, by definition, the editors create real books (e.g. most of the reported bugs and feature requests in this direction are still open, partially because MediaWiki doesn't provide a way for users to "organize a set of pages as a book" - hopefully this will change soon).
On those (non-Wikipedia) projects, the need for "collections" of pages (as opposed to PDF versions of single pages) is a lot greater, despite the current lack of integration, and this need could make a difference in the numbers when comparing the proportion between PDFs of single and multiple pages.
When revisiting the "book metaphor", please take into account this Wikibooks' blog post where it is discussed the confusing nomenclature we had at hands (on non-English projects as well) when Collection extension was enabled on Wikibooks projects.