It is because it applies as a layer effect (fx). I have noticed the same with other tools that initially apply a fx The thumbnail in the layers dock is the same.
Once applied both thumbnails change to the effect. This one applied to a selection.
edit: With a fx still active a colour picker work-around might be a new-from-visible top layer.
Once applied both thumbnails change to the effect. This one applied to a selection.
edit: With a fx still active a colour picker work-around might be a new-from-visible top layer.