I have a similar question, but much greater variance. I want to take advantage of apple's battery replacement program. I know functionally my battery capacity has diminished a lot. Have iPhone 6 Plus, almost 3 years old. Coconutbattery says I've had 674 charge cycles and current charge capacity is 35-36% of design capacity. Apple's new Battery Health (beta) says its 83% of maximum capacity. Based on how quickly it discharges and how often I have to charge now vs when it was new, I'd say cocunutbattery is closer to the truth. But how can they be that far off?