As in the real world, a horse’s performance is not always consistent. "Everyone has bad days" and horses are no exception. The Ability Variation is an attribute that is randomly assigned at birth and can result in unpredictable positive or negative racing outcomes. The value of this attribute will determine how high an impact the random variation can have on the horse's regular performance.
A higher ability rating in the parents will result in a smaller chance of a negative Ability Variation that will impact the racing performance of the foal.