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 a

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.