I think it’s because it’s pulling the actual raw data at the time and that data can and is changing quite often.I know they have to try pulling from Apples own stats. But the numbers don’t match with Apples own metrics consistently.
I’m guessing what apple does for their magic battery health number is averaging the max capacity of those raw data numbers over time. And of course they are going to show the user the highest possible number at all times even though the actual capacity is always changing because of environmental factors such as temperature.