For women who have low body fat it will be more irregular because of insufficient estrin provided by adipose tissue. A woman with poor diet and low body fat would be irregular.

Spring season is a time of abundance and fertility for plants and animals so I imagine Hunter gatherer societies had a better diet during spring and therefore women met body fat requirements for regular menstrual cycle and ovulation.

Maybe the 4 linear marks in the cave painting are for the 4 months after winter solstice and 14 dots are for days to ovulation after menses?