If a customer is on a pack where they get x amount of credits, and the pack has an eventual expiration date, the customer may use up all of the credits prior to the expiration date.
In this case, they won't show on the pack details page in the 'active' list -- because they've used up all their credits. Nor will they show in the inactive list -- because they're package is not yet expired.
Once the pack's expiration date passes, the customer will show in both the 'inactive' list on the pack details page, as well as in reporting.