I don’t think the power shown on the card is very accurate. It is common that the strength of the 5s compared to its comparable 4s lies in its potential to get to level 80 on last ascension and is otherwise only of similar strength or even weaker.
As comparison: I have Grimm and isarnia both on level 70. Isarnias power is 680 compared to Grimms 645 but her health and attack is lower and her defence similar. Her special is stronger but this is negated by her being slow where Grimm is average speed.
All in all, I can’t see that she is more powerful than Grimm at that level. Devs has more or less said they added a “rarity bonus” on the cards power. Details About the Team Power Change: