Human legs are never of
exactly equal length; you may believe you walk straight ahead, but slowly, imperceptibly, your course will curve, you will tread a circle, and your beginning will be your end.
And I'm assuming he's comfortable simplifying to case where «months» are actually
equal -
length periods, e.g. 30 days and redefining annual as 360 days, or each month
equaling exactly 1 / 12th of a year.