Can you be more specific about the families you are talking about?

My understanding is that it is determined by the geometry in the family projecting a bounding box of sorts in the project. Most of my families I work with are fairly rectangular with their bounding box, but if you have elements that overhang or extension at different levels that would cause different results I would imagine. But I think of it as a bounding box created from the geometric limits of the family, and that is for all model elements including solids, voids, model lines, ref lines/planes etc.

Maybe give some examples of elements that display unusual selection behaviour, then we can dig a little deeper into this

Ok I got it. As you said, it is determined by the geometry. My problem was I choose to deal with yes/no parameters to switch element's visibility.
Even when the geometry is not shown, it commands the enlightenment for preselection.
It could be annoying sometimes....

Yep, this is it. Except the refplanes, they don't do anything (but reflines do since they have a start and endpoint).