Hi
Ok. I've checked that and there is nothing wrong with it. It's how joomla works. Category page with this header added is a landing page to all of the articles from this category, which means every article will show modules attached to it's landing page (category page).
Workaround:
1. You can create a separate menu item for every article, so articles will have their own Menu Item ID and selections of modules for this page will be available
2. You can use Advanced Module Manager plugin which will extend module attachment options so You will be able to specify if module should be used on article pages. <<< I would recommend this solution.