Filtering a book for variants is different from searching for variants.
Filtering: When filtering, your aim is to substitute your primary object with a specific variant object. You do this by selecting values in the filter in order to match your chosen variant, and exclude the other variant objects. This action requires that, at a minimum, all possible values that have been assigned to a variant are selected in the filter.
Searching: When searching, you're looking for all objects that have one or more variant criteria values assigned.