Many other explanations are possible.
Perhaps the curvature (or perceived curvature) of the universe changes radically as you go "out".
Perhaps the speed of light is only constant relative to the local density of the universe.
Perhaps the other properties of light change with time, which affected the perceived speed differential.
Perhaps the light traveled a longer path than assumed, being deflected by something that we cant "see."
Perhaps one of the scientists had a spot of mayonaise on his glasses