I would assume it's for a similar reason to why neutrinos from a supernova arrive before the photons. Photons won't be traveling at the speed of light (in a vac

I would assume it's for a similar reason to why neutrinos from a supernova arrive before the photons. Photons won't be traveling at the speed of light (in a vacuum) until the're actually in a vacuum. Neutrinos and the propagation of gravitational waves wouldn't be slowed down as much by the matter between the center of the event and open space.