skos:Concept not appearing as class in graph view

At the right of a graph representation one gets the list of all classes used in the graph allowing for further filtering.
In the case of an individual of a skos:Concept, the class does appear in this list, however not as (skos:)Concept but it appears with the label of the individual concept (example: 'health').
In detail view it is clearly indicated that the thing 'health' is a Concept.
Any explanation?