Because even with hygiene products, people buy them before they need them and stock up. And with food, you're often shopping for a week and for the whole family. You put things on a shopping list and don't buy them the same day you're using them, or even the same week.
I suspect that even if the manager thought they could determine it, their actual "results" were entirely random. After all, how is he going to check? Call two hundred of the women and ask? Also, periods are irregular. They're not a perfect 28 days each time. They vary month to month.
I don't doubt the manager thought he was doing that. I doubt it actually worked at all, though.