Each skill begins with a certain amount of variance based on a number of factors, such as how easily the skill is mastered and how reliably it can be done. Certain skills are more luck-based and unpredictable than others, at least initially. Most people’s pain tolerance (Resilience) and level of erudition (Knowledge) are fairly consistent from occasion to occasion. But a character’s chances of surviving in a hostile environment (Survival) or sneaking past a guard (Stealth) depend more on getting lucky in a multitude of small ways, more often than not.
However, just because a skill is low or high in variance when the character first learns it does not mean that the character still uses that skill in that way. Everyone learns and uses the techniques of their skills differently, and individual proclivity can reduce or enhance the randomness inherent in trying to use a skill. Characters can pay to either add to, or subtract from, the number of Variance Dice they must roll for each skill they know, pushing that skill towards being more consistent or more variant.
Skills that begin with low variance tend to be skills where predictable consistency is valuable, and skills that begin with high variance tend to be less predictable but more prone to impressive accomplishments (and failures). These characteristics can be enhanced by spending XP to further raise or lower the Variance Dice. However there is nothing wrong with pushing a skill’s natural variance in the opposite direction, to soften or even reverse a skill’s natural tendency towards variance or reliability. The Pilot of a passenger craft who is routinely responsible for many lives would want to push the naturally-high variance of Piloting as low as possible to avoid the potential for disaster, because there’s no need for him to take risks in order to over-achieve on a routine flight. On the other hand, an eccentric inventor might have raised the naturally-low variance of his Craft skill, on the logic that it’s better to experiment with radical designs because it doesn’t matter how many failed prototypes it takes to make one, once-in-a-lifetime work of genius.