Jean Cousin, "L'art de dessiner," circa 1768
Head to belly button proportion is for drawing children, why do you find this odd?
In fact most pictures want something, life-size baby drawings are cheaper because of the cost of the drawing paper. Kids who were all too often overlooked and unseen.
What did you enjoy doing at age 4?
As a rule-of-thumb, a good way to deal with human proportions is to ignore the rules.
Jean Cousin, Author of seventeenth century drawing tutorial