Batteries have a limited lifespan. Every time you charge your device, it wears out the battery, lowering its total capacity. Scientific research shows that battery lifespan can be extended up to 200%, when you charge your device to only 80%.
AccuBattery measures the actual battery usage using information from the battery charge controller. Battery usage per app is determined by combining these measurements with information on which app is in the foreground. Android calculates battery usage using pre-baked profiles that device manufacturers provide, like how much power the CPU uses. In practice, however, these numbers tend to be highly inaccurate.