Selection.activeObject to Selection.objects changes MonoBehaviours into GameObjects?

I'm selecting a MonoBehaviour instance by assigning it to Selection.activeObject...

Code (CSharp):

1. Selection.activeObject = someBehaviourInstance;

Later on I add another instance of that MonoBehaviour to Selection.objects...

Code (CSharp):

1. List<Object> selectedObjects = new List<Object>(Selection.objects);
2. selectedObjects.Add(someOtherBehaviourInstance);
3. Selection.objects = selectedObjects.ToArray();

...and now the Editor says that I have selected 1 MonoBehaviour and 1 GameObject (same if I read Selection.objects). MonoBehaviour inherits from Object, so I don't see a reason to change into a GameObject. Is it a bug, or am I doing something wrong? (Unity 5.6.2f1)