This confuses me, because I've heard from many that yeast does not perform at its best until fermented in a batch of beer for a generation or two (and also found it true in my own experience).
Making a starter and making a batch of beer are different goals. The goal of a starter to take a small amount of yeast and grow it into a larger amount of yeast (i.e., increase the yeast biomass) while protecting its health. Time and time again, I see people commenting on the improved performance of the the shaken, not stirred method. The magic is not in the shaking. The shaking is a poor man's O2
bottle and diffusion stone. The magic is the result of not stressing the cells and pitching at high krausen.
With that said, the reason why culture improves when repitching is not based solely on number of generations the culture has been propagated. It is based on the number of generations under real-world brewing conditions in one's brewery. The culture improves because the wheat gets sorted from the chaff cell-wise, and the culture becomes acclimated to the environment. It's Darwinism applied to brewing.