Why do some women have such an easy, quick time giving birth but others labor for hours?

I’m very lucky, just a few hours every time, from beginning to end, and very little horrifying pain, except at the end. My Mom was the same way, but she didn’t know it.
But I’ll tell you who takes the cake out of everyone I know and that’s my Daughter-In-Law. It’s like, “Boom boom,” she’s done. AND walks out of the hospital in her pre-pregnancy jeans! You wouldn’t think it looking at her. She’s tiny, maybe 5’2”. I don’t think she weighs 100 pounds soaking wet. She doesn’t have a lot of womanly curves, either. She’s the last one you’d think, “She’s built to have babies!”

I listen to other women’s labor stories and sometimes it’s clear that some of them wouldn’t have lived through it a hundred years ago.

Why is it so easy for some, and so hard for others and there doesn’t seem to be any rhyme or reason for it?