After 14 months, the average iPhone will be at 86%, as it will lose about 1% per month.
A battery is a chemical device, and chemistry is generally pretty variable and uncertain, as well as being analog, not digital. Apple specs the battery capacity to remain above 80% for 500 full charge cycles, but that is a minimum requirement; there is no published maximum expected capacity. So sometimes batteries will perform much better than that minimum specification, and sometimes the change in maximum capacity won't be linear.
All iPhones have a specification for the battery. As an example, for the iPhone 14 Pro that is 3200 milliampere-hours (MaH). So the battery monitor is calibrated for 100% at that value. But there are variations in manufacturing, so some batteries will have less capacity, and some will have more. Suppose your battery had, say, 3520 MaH capacity (10% over standard). That would still show as 100% (even though it was actually 110%), but as it aged the health would stay at 100% until it fell below 3200 MaH. This would appear to you as if your battery had fabulous life, until suddenly it didn’t.