I'll make an attempt at answering that, and you can correct me if you disagree. It seems that the non-uniformity of the medias involved is being made evident in such phenomena. In other words, the light-speed reduction defined for a medium is a macroscopic quantity based on averaged effects of all the atoms that make up the medium. Looking at it microscopically, the medium is a combination of vacuum and atoms, and light is normally induced by the atoms to meander about instead of taking the shortest path through the vacuum.
It's the kind of thing we don't know and may never know. Even the deepest outer space between galaxies and between galactic filaments where there is really nothing contains photon radiation fields and and gravity fields and who knows what else criss-crossing in every direction everywhere. There may be no such thing as a perfect vacuum, and if there were it might be disastrous.