A woman’s cycle varies between 15 and 45 days, averaging 28.1 days, but with a standard deviation of 3.95 days. That’s a hell of a lot of variability from one woman to the next. And the same variability can be experienced by a large minority of women from one period to the next, and among nearly all women across the course of their fertile years.
On the other hand, the moon’s cycle (as seen from Earth) takes 27 days, 7 hours, and 43 minutes to pass through all of its phases. And it does so like clockwork, century after century.
Of the two, I am finding the second to have a much stronger likelihood of being the reasoning behind the notches.
Strange how gender-bigotry style historical revisionism and gender exceptionalism seems to get a wholly uncritical and credulous pass when it’s not done by a man.