Seems more likely that it would be the voltage rating? While it’s true that a larger capacitance would be fine in many applications, it wouldn’t always be (e.g.

Seems more likely that it would be the voltage rating? While it’s true that a larger capacitance would be fine in many applications, it wouldn’t always be (e.g. if the cap is part of an RC filter or if it’s used as a bypass cap). In contrast the voltage rating of a cap is just a maximum.