I've often thought about this question. I find it interesting.

One example I've got that I think is relevant is a 5 mile race I've done twice. The first year I finished in a real state, hardly able to talk, slightly dizzy. The next year, I was a couple of minutes quicker and not in such a state at all.

I wondered if I'd just not pushed myself as much, but now I more think that it's just that I was fitter, so my lungs were more able to cope, but the pain had gone from being mainly in my lungs to more in my legs. So even though I wasn't in such a bad state at the end, I probably had pushed it as much as I could. It's just that my lungs being in a better state they gave the impression that I hadn't.

That's the interpretation I'm sticking with for now anyway!