While I feel 90% is excessive, battery health monitors need to be taken with a grain of salt especially the battery health monitor in an iPhone. Its literally impossible for a battery health gauge to accurately show a single percentage without a large portion of luck. There are far too many variables, far too little data, and no where near enough time.
Every battery manufactured has a slightly different capacity. However Apple just uses there manufacturing spec across the board for their battery health monitor. That is why some people have a battery that stays at 100% for months. Its not because its degrading slower its because its capacity when new was above Apples 100% spec.
Apples algorithm is programmed with a bias. It never increases! This is to prevent customer confusion and tons of PR however battery health sliced into single percentages isn't enough time to factor out all anomalies. If there is an extreme variable in the data it can have a larger impact on the data set, and if that variable reports 90% when the health is 95% what it will do is sit at 90% for months until it calculates 89%. For example 4+ months and 64 battery cycles later and my battery health has increased!
Battery capacity degradation isn't linear in respect to time when applied to average smartphone use. Since lithium i batteries are overcharged for higher capacity to approx 4.2 volts day in and day out they will degrade faster toward their nominal voltage the further they are from it. So the decline from 100% to 90% is faster than the decline from 90% to 80% everything else being equal. Smartphones in particular have capacity degradation that gets exponentially worse, the battery has a lower capacity and needs to be charged more, charging more increases wear and tear though cycles thus having to be charged more. Point of mentioning that is people in general correlate percentages with time when it comes to their normal routines.
But then we need to consider the battery health percentages statistic overall. At 79% you will get a popup in iOS that states "Your battery’s health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity.". At 30% you will be lucky if the iPhones current to start up doesn't cause a low voltage shut down trigger. Why would an Apple battery health monitor on Apple hardware using an Apple battery report there is technically 30% battery health when there is obviously 0% of capacity accessible to the iPhone.
Those are just some issues with the battery monitor system itself. The data it has to work with is a different story.
Using CoconutBattery I can see the the fluctuation of battery health between summer and winter. Since a batteries chemistry is slowed by cold temps the capacity temporarily decreases. Do this enough times and the average decreases. When spring comes around it starts trending back up.
Some people don't allow their battery to get fully charged while most people dont let them completely die. While this is good for the batteries health its bad for the batteries health monitor. Not letting the battery die makes the battery life gauge and health unpredictable, I think we've all experience a device that either died at 10% or got to 1% and lasted for another hour. This is just because the effect of higher current from lower voltages were unknown. Conversely a factor used for calculating capacity comparing the normal charge rates to "tickle" charge at as the battery reaches higher voltage to see how resistant it is to accepting and holding the charge.
I could go on and on but looking up and this is getting a bit insane even by my standards...
