Inheritance to first born

With Corona my mother in law has been talking about her will if she dies.. she was going through her list of who will get what . When it came to the grand children she said the oldest girl gets the majority of her jewelry because she was the first girl.. and the rest get 1 piece.

Why do people do that? Why not treat all grandchild equally ?

I obviously didn't say anything because i respect her decision- it's her stuff and she can do what she likes with it but I can't but help feel it's not fair to the other grand children. Just personally I would always make sure I gave each person the same amount of stuff regardless of who was born first.