Why iphone is not showing accurate battery percentage?

When i slept tonight it was 60 percent, i got up in morning its still 60 percent, i plugged in charger, suddenly it became 63 percent, why?
Please tell if i discharge it completely & recharge to 100 percent wil it help to calibrate?
Or factory restore is must to calibrate?
Please you know factory reset means all my apps will be deleted, what to do?

There's a fault that has to be checked by apple.