There are bugs in the power that is displayed. Some times it displays more that what you have and some times less but this does not affect combat. A refresh usually fixed the display to your actual power, but not always. Other thinks that forces a power recalculation and correction of the number are taking runes off and then back on, taking heroes out of the formation and then butting them back on and taking equipment off a hero and putting it back on.

If you are in the top 50 in the arena, I have found that the power stated there is usually accurate, and as Herby said, fight something and see what the power is during the fight. That is always accurate.