Hello,
In your widget settings, you've set order by id DESC, and you've provided 8 category slugs, so widget takes posts from these categories but by post id. e.g. post1 - id = 1(category1), post2 -id = 2(category1), post3 - id=3(category2) - in this case widget will display posts by id so post1 and post2 from first category, then post3. You can choose another "order by" option to change that.