Air that's "naturally" going away from diffuser is being used to seal the floor with floor side slots. Rear tyres are driving loads of air on top of the floor outward, acting as huge air dams.
This CFD post from Vyssion clearly shows the trend in front of rear tyres. Fences on the inside of those tyres are there to increase pressure in that zone, thus feeding the underside with more air and seal the floor. Also, a vortex forms on top of those fences, I think it helps with diffuser sealing as well. Vyssion's model doesn't have floor slots, yet trend is the same as that flow viz paint.