A skull is a vital, functional, evolutionarily important trait, and its development, as that of all complex traits, involves the interaction of many pleiotropic genes and modifying factors, most with small effects, interacting with environmental influences.The idea is that an organ so complicated as a skull is the product of many genes each influencing multiple phenotypic traits and at the same time being influenced by external factors. In other words, there is no such thing as one "skull gene".
Buchanan et al. 2009. What are genes "for" or where are traits "from"? What is the question? BioEssays 31:198–208.
When multiple genes and multiple environmental factors each have a small contribution to the formation of a skull or any other organ, continuous quantitative traits such as dimensions associated with that organ usually display a normal distribution.