Your explanation, AYS, was full of insight, wise words of a participating parent.
I could also see where a teenager just all of a sudden clicks, and gets their competitive gear in full drive. I guess it might be hard to believe after hours, no years, of training, but as a small point I remember when one of my kids clicked about kicking the soccer ball to where the teammate was running, rather than to where she was because that would end up being to where she had been if all were running forward and many kids kick the ball behind their intended target.
So do swimmers and track or any athletes improve their mental game as they go along, I think so.
However, having said all that, with the Chinese swimmer in question, what astounded me (and again I could be naive here) is that I thought she was humming along just exactly at more or less the same speed as the others and then she exploded in the final lap, where all other girls were running out of steam on the last lap, she created power out of nowhere, do swimmers really have the ability to find 5th gear? That last lap was suspicious to me.