I've never quite understood this and I can't seem to find an exact explanation anywhere. If I charge my iPhone up to 100% and take it off charge shortly after it's reached 100%, the battery will drop much faster than if I were to leave it on charge for a few hours after it's reached 100%. For example, if my iPhone's been on charge all night, in the morning when I start using it the battery meter will remain at 100% for a couple of hours before dropping to 99%. But if I charge it during the day and take it off charge shortly after it's at 100%, it'll drop to 99% within ten minutes of use. So in both instances the battery meter reads 100%, but one will last a heck of a lot longer than the other, even though they both start from 100%. What gives?