Battery Maximum Capacity

Battery Health & Performance 

I see that you have an iPhone 11. Your battery health is fine and is undergoing normal usage. iPhone batteries, like all rechargeable batteries, are consumable components that become less effective as they age. You should change out the battery once your Battery Health reaches below 80% which will take over 2 years to reach under normal usage. Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➔ iPhone Battery and Performance - Apple Support

Axel F.  

91% is still good, your battery is a consumable and as you use it that battery health will decline. It may not do so in a linear fashion and it is affected by a wide variety of variables. When your battery reaches 80% health it will be time to replace it.