Neutrinos vs Photons
What frequency were the photons?
It's possible photons of different frequencies have very very small differences in the speed of light -- which, given the huge distance to the supernova, equates to 7 hours difference travel time to Earth vs. the neutrinos.
This tiny difference would not be measurable in an experiment involving measuring the speed of light across distances many magnitudes smaller.
eg. If the largest distance we could measure the speed of light over was from here to Pluto (around 0.0007 light years) we'd need to measure the distance (assuming our measure of time is exact) to within 4x10^(-9) metres to be sure the speed of light we find isn't sufficiently different from neutrino speed to lead to a 7 hour delay from the supernova 1987a (168,000 light years away).