I believe they're testing different ways to make the game overall smoother, even at a slight cost to FPS. Somewhere in the
beta-patch forums one of the devs said they especially want feedback on how smooth the game feels, and that benchmarks don't necessarily represent this very well. I noticed precisely this effect with one of the earlier beta-patches: smooth gameplay but slightly lower FPS. I haven't tried the latest ones so no idea on how they've progressed.