I mean, we can break out luck to be all the various factors. But it can be summed up into one (for the purposes of this discussion). And each of those will still have some mean that they tend toward. The weather, outside San Diego, isn't going to be perfect year round. Your relationships (impacting mental state) won't all be rosy. Your sleep can be interrupted by a neighbor moving in at 3am. Though that last one is atypical. And if it happens, and your race time is 8 minutes instead of the average of 7, you should still expect to be closer to 7 the next race barring similar misfortune.

I think the main point is that performance depends on some factors that we can reasonably expect to remain constant between races ('ability'), and others that are likely to vary quasi-randomly between races ('luck').

Those names are imperfect but the underlying logic is sound: the winner of a race is relatively likely to have both high 'ability' in himself, and higher 'luck' than usual for that particular race. In the next race, his 'ability' will remain high, but he will probably have roughly average luck.