"My experience of you" vs. "real you"

It's funny once you become aware of something and it's on your radar, you start both (1) seeing other instances of it and (2) you understand what's going on in those instances. For instance, I remember at one time in my life not understanding the meaning of the Fleetwood Mac song Landslide, and I also remember there being a very specific (although I've forgotten it now, ha) moment in which I suddenly understood it and it applied perfectly to my situation at that time.

I've always liked this Bjork song, but a few months ago I finally understood it:


I watched the first episode of the Netflix comedy Grace and Frankie, sort of an odd couple dynamic between two women whose husbands leave them for each other. Frankie is hippy dippy, Grace is rich white lady. But it also had an example of the sort of defining someone's identity that I mentioned in the last post:

Frankie: I lost my best friend. You don't even like Robert. 

Grace: You have no right to judge me. You don't know us. 

Frankie [clears throat] I'm sorry, I was judging by my experience of you, not the real you. That was wrong of me. 

I thought, that's a good distinction to make -- judging by our experience of a person versus whatever the real them is. We would never assume that we know all there is to France and French people after watching a French film or visiting Paris. Why do we feel so sure of ourselves in terms of our ability to judge someone's character after seeing a similar small sliver of the real them.