Because they know us VERY well and more often than not, a smart strategist who knows his opponent inside and out can beat him if that opponent relies mostly on emotion, energy and continuity to win.
McVay knows what buttons to push to get the W. We win when our emotion elevates our execution and we just out-will and over power them. Otherwise, he makes enough cuts to bleed us to the point we lose our edge, and then we are done.
Pete's teams are all about energy, emotion and togetherness elevating xs and os that arent overly complex. It works when the guys come together and play as a seamless piece. If they dont, its fits and starts, ups and downs, and we are susceptable to superior strategists.