If APM finds the articles, assigned to pages, it processes first those articles. Therefore, assuming your APM session refers to the News section with 25 pages, and you assigned articles to page 11, 15, 21 and 32, the APM engine will first process those pages and place the assigned articles, according to the prerequisites explained in Making sure an article will appear on specific page.

If you assigned, for example, only one article to the page 15, but the library object selected by APM contains more logical groups, at the very first moment the additional logical groups will remain empty, while APM is processing other assigned articles.

As soon as all assigned articles were processed, APM reprocesses all the pages with the assigned articles, and fills the shapes remained empty.

When that's done, APM proceeds to paginate all other pages in the current APM category, as long you did not limit the processing only to the current or opened pages.

See also

Making sure an article will appear on specific page