iPhone battery maximum capacity

I keep my iPhone 11 Pro Max up to speed on iOS updates.

My wife’s iPhone XS Max is rarely updated.

We have the same amount of storage.

I updated to iOS 7.5 the same week as its release. My wife did so only recently.

We both use our iPhones for the same purpose and roughly time each day.

Although this may be a red herring I was surprised to see her year and a half older iPhone with a battery condition far superior to mine.

My maximum capacity is down to 86%, hers is 93%.

Her iPhone is my old one. So the setups on both are very similar.

where has all the power gone?