Why do some patients have small mandibles other than besides genetics ?

Syndrome / variation: Hypognathia, underdeveloped mandible, when isolated and of mild degree is usually not associated with other major congenital anomalies. It may be seen inherited in some families. Many syndromes with significant hypognathia are known to occur and affect brain, other organs or skeletal structures.

It's almost alway: genetic. Some babies are born with a markedly small jaw but by one year of age the jaw usually catches up to normal.