User blog comment:Derax-Pagani/Chapter 507: Voice/@comment-5708675-20161025153003/@comment-3045936-20161027214330

Brandish corrected Makarov with August. She does acknowledge that August is the strongest of the Spriggan and the strongest male. But she said that if August is the strongest male, Irene is the strongest female. The reason besides knowning him as a child, she tired to negotiate with him, as she said besides Irene no one can stop August.

With that said August is the strongest of the Spriggan. That isn't questioned. But Irene's power is near his level. To where even though August is technically the strongest doesn't mean Irene couldn't defeat him. It's all the variables. Like with Ajeel and Ezra. Jupiter Cannon, distractions and the fact Ezra is one of Fairy Tail's strongest is why she won.