Human hand preference is predictably a biological and genetic (hereditary) phenomenon, but we see that 85 percent of people are right-handed.
Researchers have identified the specific gene (D Gene) that occurs more commonly among the population which responsible for promoting right-hand preference. The C gene, which is less likely to occur within our gene pool, creates a coin-toss chance of an individual being left or right-handed.
Just like the weather, nature made sure that there are enough influencing factors that cause changes in hand-preference, like external, societal and cultural pressures, to result in various possible genetic combinations and chance influences.
This makes the entire process of hand preference unpredictable by design.