I do think there's some manufacturing variance from battery to battery. From what I understand, iPhones will stay at 100% for along time because the percentage is actually over 100% (the actual charge capacity is higher than the "rated" capacity). Then, they'll start dropping below 100% and will continue to. In reality, the percentage was always dropping, you just didn't know it because all you saw was "100%." Some iPhones, fresh off the production line, have 105% battery, some have 110% battery.
YES, usage and good/bad charging habits will also impact battery health. But I believe the above paragraph is also true.
I believe that's why some people can do everything right and have 95% gone after 6 months, and why others can adopt the same charging habits and still be at 100%.
FWIW, I've had my iPhone SE for a year and it's down to 92%. It was at 100% for like six months or more. Been falling like a rock ever since!