That looks to me like there is a reduction in angle of those tufts, it looks like the tufts are being sucked downwards towards the sill more without the curtain.
The curtain is blocking the downwards flow? Or there is the possibility that the wing shape is creating a vortex that acts to "seal" the underside of the car, blocking the flow. It looks like any wingtip vortex would be the correct direction to reduce airflow under the car.
As in moving outwards at the bottom and inwards at the top.