Sorting of pages?

[Carlos]

I don't even know where the value "sort" comes from?
It is a mixture of menu-title and something else.

It's "Parent title + Page title", see admin/pages.php line 75:

Code:

$sort = $parentTitle .' '. $page['title'];

so that the (backend) list is sorted by top pages, then subpages, etc. (tree-style)

Menu order is used by the frontend function get_navigation.