I've noticed that too. I have ADHD and I can tolerate a lot of pain.
I don't know if its tolerating it so much as ignoring it. If I'm in the middle of something and cut myself or drill a hole in my hand, I may not even react to the injury unless the blood is interfering with my work. Yeah sure. It hurts, but not enough to stop me doing whatever interesting things I was doing when it happened.
We have difficulty voluntarily focusing on things. Whatever controls what we pay attention too simply doesn't rate pain as interesting enough to focus on at the expense of the rest of the world and whatever is going on at the time.
Time doesn't register with me unless I think about time. If I'm doing something the time can go right by even if there's something else I was supposed to be doing. It just doesn't click. I think pain is the same way. It's just there, but it's not really important right now.
When my wisdom teeth came in, they came in sideways "Impacted" the dentist called it. They were pushing on my back molars from beneath the gum line. Anyway, the back molars both cracked. It hurt a lot when I took time to think about it, but I went over 10 years before I finally had the wisdom teeth and the ruined molars removed. It just didn't click. Time flew right by. If I didn't know it was 10 years, I'd have said it was a month or two, I think.
This next part is kind of gross so skip it if you're squeamish.
My kid said she had a headache one day. I gave her some motrin and she was fine. Then about a week later, she told me her ear was clogged and she couldn't hear very well. We used alcohol to try to clear it out, but she said it didn't work. So I took her to the doctor. It turned out she had an ear infection that had burst her ear drum. She never even complained about the pain at all! From what we can tell it had to have been going on for a few weeks.
That same year, she had a lump on her back that turned out to be an infection that the doctor had to drain. She stood there hugging me, while the doc used his hands to pinch and poke the thing to drain the stuff out of it and clean it. The whole time she was just humming a little song and looking at the posters on the wall. You'd never know anything was going on by looking at her face. The doctor looked at me and whispered "If anyone did that to me, I'd be howling right now." We were very impressed that she didn't react at all. She said she felt it and it hurt, but that's it. We told her she's "Viking Tough" because of it. Now we know if she says something hurts we need to take her to the doctor.
The funny thing is, she'll suffer a burst ear drum and an infection on her back in silence, but she won't drink medicine without acting like someone took a branding iron to her.
Kids are amazing.