When filtering list vies, lookups and option set fields allow user to filter on data that isn't even available in the current view. Filters should only show values for records that are in the view to streamline the user experience.
For instance, option set called "Color" has values are Red, Green, and Blue. The current view only has records for Red and Green. When filtering the view, user should only be presented with Red and Green to filter because no records have Blue.