The military does not survey the population and then select the fittest. So, as a function, it cannot actually perform as you say.

It's the same with F1. "We have the best drivers in the world!" You have the best drivers from the self-selection mechanism you impose on the sport. There are zero reasons to think these categories have good overlap.

They don't need to have sampled the entire population to have ended up with some of the most x individuals of the nation of y population size, they just need a large enough pool that the top end up among some of the best.