What's happening is that the battery that was installed in the phone at the factory had a higher real capacity than the specification. For the iPhone 6, the battery specification is 1810 mAh, but the battery you got (lucky you) probably had a higher real capacity than the specification - essentially giving you a bigger battery.
The new iOS 11.3 battery power management feature is comparing YOUR battery to the specification. Your battery may have degraded 5% from when it was new, but if it actually had 5% more capacity than spec when new, the power management software wouldn't know it... So it would appear to be at 100%.
If Apple wanted to be sneaky, they could under rate the battery specification and all new batteries would exceed it. Then we would all have batteries lingering around 100% design-capacity for years... though they still would have degraded.