That is how much of the battery capacity is hidden by the battery management system when the car is new and then slowly doled out as the battery ages to make for the appearance of very slow degradation even though the individual raw cells would be wearing out quite a bit faster? If this were true what you would see is after this excess capacity was exhausted would be battery capacity falling off a cliff eventually, though this data seems to show a couple hundred thousand miles of consistent capacity with no cliff.
SSDs do a similar thing for capacity and wear with a sizable proportion of capacity reserved to replace bad blocks as the SSD ages.
Whenever I make this comment almost everyone responding is just guessing about how I'm wrong and new chemistries are so much better, etc.