The power listed in the rankings chart is the highest power you've ever had.

Things that cause it to be higher then normal for short periods of time.
-Putting a purple hero with a high saint ranking into your formation temporarily.
-Replacing low/no power runes around. Take out a Righteousness and put in say Advantage.
-Swapping gear between your high power and low power characters.
-You may have lost some star levels recently.