It's also unknown what percentage of Liberation's volume is accounted for by its PlayStation Vita bundle. We've reached out to Ubisoft for clarification and further information and will update when either is received.
Update: Ubisoft has confirmed with us that Assassin's Creed 3: Liberation has shipped nearly 600,000 units. For clarification, the term "sell-in" refers to units shipped to retailers, while the term "sell-through" refers to units purchased from retailers by customers. Thus, Ubisoft (or anyone else, for that matter) can say units have been "sold" when referring to either statistic.