Why can't I get 100% charge anymore?

I keep my Ipad 4th generation model A1459 on my nightstand 99% of the time and leave it connected to the charger. It has been staying at 100% full charge. Some days, when I take it off and use it, it drops to 90% charge and then goes back to full charge after being connected to the charger. I noticed today that it is only 93% charged even though it has been connected to the charger for several days. What could be the reason it is not fully charged ? Could this be a problem with the charger or the battery ?