For RF lenses, more of the light that strikes the lens ends up in the correct place on the film. There is less diffusion. The OVERALL amount of light at f/2.8 is theoretically the same. But correct placement contributes to better sharpness and higher contrast, hence the high regard for RF glass.
Actually few lenses hit their stated apreture f/stops accurately. Cine lenses, which need high tolerances, use "T/stops" which are set lens-by-lens by actually measuring transmission for individual lenses, rather than trusting the theoretical numbers printed on the barrel by the mfgr