I thought that a vacuum is never 100% because of background energy or something, so I could imagine that neutrinos are less interactive and so they live in a better vacuum than light. But I also read that we've seen a neutrino burst coincide with a photon burst from a supernova in 1984 or something, so the photons and neutrinos seem to have the same c value in space.
That's why I'm guessing gravity/neutrino interaction, since it has to do with a near-earth effect.
If I'm right, you heard it here first! =)