Attack or Defense

I have a question: Does anyone know why the developers treated the owners of the Defense specialization (with 5%, 10%, and 15% bonuses) so unfairly compared to heroes with the Attack specialization (10%, 20%, and 30%)? Why is the percentage bonus for Attack twice as high as for Defense? What is the reason for this?! And in general, is this an imbalance (especially since even the Defense specialists are twice as weak)?