Is my extensor tendon supposed to slide over my knuckles?

I'm (22m) and I just learned about boxer knuckles, from what I learned the doctor was talking about extensor tendon sliding over the knuckles, and mine has always done that since as long as I can remember. Never had any complications. Is it normal for it to slide over the knuckles? My sister's doesn't. My brother's also do that, only on some fingers, for me it's most of my fingers.