Yeah, I'm not understanding this either.
I jump around between 25-30 FPS in a small fate, regardless of resolution.
AMD FX-8120 @ 4.2GhZ
Radeon HD 7870 X2
16GB 2133Mhz Memory
Sabertooth 990FX 2.0
Corsair Force SSD
Getting the same FPS @ 5760 x1080 and 1920 x 1080
Enabling my second 7870 gets me a 3-5 FPS increase.