Searching and applying filters in Dimensions will always filter documents with the other attributes of those documents still being present in a search. For example, when filtering for a research organisation in publications, researchers from other organisations will still feature in the results because they are co-authors on the resulting publications. (For the same reason, filtering to an individual researcher and then going to the "Researcher" tab in the analytical views will present not just the selected researcher, but also anyone that they have co-authored publications with).
In order to produce a list of only researchers from a certain research organisation that match your search results, simply navigate to the "Researcher" tab of the Analytical Views area, and then select the checkbox marked "Show only researchers who currently work at/in: (research org)". This will reduce the list of researchers to only those that have an affiliation to the research organisation to which you have filtered (see below for example). Please note - this feature is not available in all Dimensions versions.