I have an iphone 6 (not even 6s), with the original battery. The battery health says capacity is 96%. Maybe that's a poor estimate? There are some things that run down the battery at a fascinating pace, but that's a very recent event. For instance, I can lose 20% charge in airplane mode. I now have to turn the phone completely off when I want to store it somewhere like in a locker when I work out (yeah, I'm a boomer I know- I'm one of the few people in the gym who don't have headphones on).
Definitely a bad estimate if that is an original battery. 500 charges should bring the health down to about 80%. Presumably yours has been charged 1000-1500 times since it was purchased new from Apple.
My 11 Pro is at 96% and it is only a few months old.
Lithium battery degradation is a chemical fact; it happens as reliably as, say, car brakes do with use.