Hero Powers have been adjusted slightly in the new update (version 15.1). The change only affects the values of Hero Power - the actual effectiveness of the heroes is completely unaffected.

Higher rarity heroes start out with a slightly lower Hero Power, but the Hero Power increases more quickly as you level your heroes up. In practice, the Hero Power of fully levelled Legendary heroes is about 10 points lower than before.

The motivation for this change is to make the Hero Power reflect the actual power of the heroes more closely, especially when dealing with heroes that have not been levelled up yet. We hope this change will make it easier to gauge the power of your potential opponent, as well as improve the matchmaking for Alliance Wars.

Again, please note that the actual effectiveness of the heroes is unaffected by this change.
For further information and discussion, please visit this forum topic and post.