I have noticed this as well, just thought maybe a player's win streak modifiers were carrying over to their defense which is awesome if it does. There is definitely some kind of hidden boost applied to Arena defenses because you can set up the same defense as one that is giving you trouble and when you test it, you can easily annihilate it when the same exact configuration will still be harder to beat in arena. I'm not complaining about that at all though, just hate the fact that you can't test a defense to see how it will perform