Part 2. FO Parameter Reference - Pagination and General Styles - Understanding XSL FO Margins - Item 'H' in the list before Table of Contents.
There is: "The sum of the page-master margin-right and the region-body margin-right. In DocBook, the region-body margin-right is zero by default, so this is simply the page-master region-left."
There should be: "..., so this is simply the page-master region-right." If not so then a problem is in the item 'G' which is about margin-left.