Underneath the hood, the collections extension when generating a book will attempt to get all the images and contributors of the articles inside the book and create sections for them.

e.g. It will make a query akin to:
https://en.wikipedia.org/w/api.php?titles=Kitten|Planet|PHP|Dinosaurs|Main_Page&pclimit=max&redirects=1&prop=contributors|images&action=query&format=json

However both the contributors and images properties are limited and it's very possible and common that the contributors and images for a single page will exceed the limit resulting in an incomplete list and thus section for the book. The limit for page contributors and images is 500

To extract all the contributors accurately we would have to make at least max(i,c)/500 queries where i is the total number of images and c is the total number of contributors (across the book).

What shall we do about this?