My guess is that, from the factory, Apple's firmware doesn't actually charge the battery all the way up to its 4.2V "full" threshold. It's probably stopping at 4.12 Volts, or something like that. Then, that threshold will slowly rise over the years in order to keep perceived battery life consistent. Eventually, after several hundred (or maybe a couple thousand) cycles, the threshold stops rising at around 4.2 Volts, and that's when you'll start to see the "battery health" number start to decline.
While I'm not an Apple engineer, I am an embedded systems engineer. I promise you, this kind of trickery is commonplace in consumer electronics. It's also far more common in expensive stuff (phones, laptops) than in cheap stuff (power banks, vapes). Cheap stuff could do this, it's not hard, but the people making those devices don't get paid enough to care.
Point being: A lithium ion battery's capacity is reduced every time you charge it - sometimes by only a couple mAh, but still. This is intrinsic to the chemistry. Your phone is doing things behind the scenes to mitigate that wear, but wear still happens. If you intend to keep your phone beyond its designed 2-3 year lifespan, it behooves you to keep charging current down.
