Missing Behaviours still process as if they exist?

If I delete a script, all the objects that had a component of it will warn about missing behaviours. I would think the functionality of that component would be lost. But in Unity I can see placeholders for variables that match my script, and sometimes when I play, I notice the missing behaviour functioning as if it was still there. What's the deal with that?

Does this mean for other assets like textures, there is a significant difference between Null and None?