It comes down to how the bellypan and difusser transition into the boxed cavity.
I saw this at Bonneville Salt Flats:
I could've got a better angle but I never saw it again. The skegs flare out from inside the tire to flush with the outer face. I think this would keep the turbulence in the wheelwell from migrating into the underbody flow, and provide fencing to the difusser.