Posts from all categories showing up on Blog page

In a site with the Minamaze theme have created a Blog (Content Top) page, marked to display Posts from one category. But what happens is that all posts from all categories are displaying. (Previously, this site had used only Pages.) I have tested by attempting another Page in Blog (Content Top) format, with a different category, but the same problem there, all posts from all categories are published. What could the problem be?