One particularly fascinating comparison is between her and my great niece, who were born about a year apart. That lets me compare them directly against each other. The differences are quite strong.
Last year at this time, Hers Truly was full-tilt into the naked romp and signing up a storm. She had just started using her words. How active and engaging she was. Even around family, she was quite the romper.
I don't see the same in my niece. Part of that is just personality.
On the other hand, Hers Truly is not like Xpioti's little wonder who loved dashing between swinging swings. I just don't see Hers Truly doing that.
You get so used to your own, and what your own does, that somebody else's child is just so ... DIFFERENT!!!
I can see how people love being child psychologists. They are so danged interesting.