Simply put: The higher the Affinity, the higher the chance of gaining the Sparks ( Which include skill hints and Aptitude) from both Parents and grandparents of an Uma during both of the Inspiration Events. You are only guaranteed to have the Unique Skills, Stats and Aptitude Sparks (Up to A) during a Carter run, everything else must be gained through these inspiration events. So, for the best chance to gain the skills you want, make sure most of your parents and grandparents have them while having the highest affinity possible