Hi Antonio,
If you only wish to display results from a specific category, then instead of using a filter with only a single option, it would be probably much better to simply configuret the plugin to include results from that category only. The linked documentation will help you with that.
The reason, why you see posts from both categories, because by default the filters are ignored when all checkboxes are selected, you can change that here. Also, since the ‘category 2’ checkbox is not there, it does not affect the results from that category.
If you only plan to display results from a single category, then then instead of the filter, I rather recommend using the solution linked in the documentation.