I bought my iPhone early this year, the battery health was around 100% just in a space of 2weeks it had declined to 94%. What going on

Our transformer spoiled and I’ve been using generators sometime I do use light whenever I’m at work or power back

It an iPhone 12pro max
What is going on?? Have anyone experienced this? Is there nothing I’m not doing right? Is the battery faulty or the charger isn’t working….

I use a 25w charger 🔌 and it the fast one

Any suggestions I would really appreciate please