Poultry lore says eggs set under a hen on a waxing moon hatch better, and there is a further rule about setting them an odd number at a time.

Incubation takes twenty-one days, which is three quarters of a lunation, so a keeper following the rule is unavoidably hatching at roughly the same phase each time. That is the sort of coincidence a tradition can build a rule on without anybody noticing the arithmetic is doing the work.