Hi Tim - this can be achieved by adding the Assessment Area filter to the context. Just right-click on the filter and choose "Add to context".
The reason you are seeing this discrepancy is that when the filter is not part of the context, the Top N will be applied before the filter, meaning that your top 15 categories will be selected for all assessment areas, and filtering by assessment area won't change that.
When the filter is part of the context, it is applied first, and then Top N is applied on top of that.
Hope this helps - let me know if you have other questions.