its to do with the balance of the cells they should be as close to the same voltage as possible this is achieved by the cells being balanced towards the end of the charge cycle
as the cells discharge then ideally they should all drop down to the same voltage at the moment the battery is turned off, but as they discharge minor cell deviations take place due to the way the cells release their charge
if a cell is failing it will show up as a voltage deviation between the other cells and what happens is that eventually that cell will have a sudden voltage drop and the battery will no longer be able to keep the drone in the air but remember what it says it will be a large deviation small ones are normal during use