Is it normal that people always stare at my gf`s hand?

Hello is it normal community,

to start this off I'll give some information about my gf.
She's a beautiful girl, but she was born without her left hand. on the left side she only has half a thumb, I don't know how to explain but I think you get my point.
When we started dating I didnt even notice it until we went out and ate something because she needed to hold the knife to cut a steak.
We are in a relationship for a long time now and also live together and so on.

But in the last weeks I noticed people always staring at her left hand. I mean is this normal? I see how people see it, talk to their friends and them turning around to also look at it. Is this normal?
I never did this and dont know why this is so special.
Is it because it is something "normal" people dont know?
Is it because it is different to a "normal" person?