Why are there fewer entries than processed pages?

A processed page may be excluded because it has a recognised noindex directive, resolves to an already included URL, has no extracted images in image mode, or was unchecked in the review list. The processed counter measures crawl activity; the selection counter measures what goes into the file.

Use the count beside the review list to understand the export. If you deselect pages, their entries disappear from the downloadable XML even though the crawl counters still reflect the requests already completed.