Hi,
The reason why you see categories in the Attributes section is because you can make each attribute category-specific. For example, if you created an attribute that you want to show only in category “A”, then you can simply set this category for this attribute and it will be applied solely to that category.
In the Listing edit page, you cannot choose more than one category because the one listing cannot be displayed in multiple categories. You have to choose one (when adding a new one).
Let me know if that helped