I think different pain tolerances, and also the size of the baby. All mine were 7lbs -7lb 12.
My first was the smallest and also the closest feeling of being to death I will probably ever be. It was all going so well until they broke my water and the pain was INSANE, the gas and air was was only just touching the pain. Had about 4 hours of death defying pain and I did want an epidural but no one ever became free to do it, then it got to the pushing stage and the pain subsided.
Second was emergency c section with forceps and I hated it and has ptsd towards births for over a year after, me and little one had bruises all over us.
The rest all just gas and air, I refused everything after the ordeal of no.2.
I think so long as baby stays the same size, I can safely say it gets more manageable. I also found counting when the contractions started really helped me feel in more control of the pain. I knew seconds 20-40 might be the worst then it would ease off and I could relax again.
Also, I refused to have my waters broken with my other children, which I think helped create less intense contractions, they all popped right at the end. Did delayed cord cutting and no injection for the placenta. Although I did need it for my last as I had retained placenta and still hadn't stopped bleeding so needed it manually removed which was another little bonus after going through the joys of childbirth.
I go through all this and my husband is still too afraid to get a vasectomy. Men don't know they've been born, women are warriors!