Even before Apple started reporting the battery health in iOS, I used the coconut battery tool to get a baseline of all the new devices we acquired. I noticed that some iPhones or iPads would get shipped with batteries that start with significantly higher capacities than official, pushing the battery health above 100% - in many cases, as high as 102 or 103%.
Because of the higher-than-anticipated mAh capacities in some of those devices, it would take close to a year of regular usage for Apple battery health to register a single drop. This higher-than-anticipated battery capacity is why many people would see %100 during the first 9-12 months of ownership. Once the battery hit the 'near' 100%, the drops would start reporting in the Apple battery health, and that is why many of us see a steady %100 for a good part of the year and then suddenly drops in %. These drops had been occurring since day one. Still, because the battery was dropping, say, from 104 to 103, to 102, and to 101, Apple kept reporting this as 100% battery healthy, creating this illusion of exceptional battery health.