One of the ways in which neutrinos are produced are through proton - proton, or proton - photon collisions. The chain of ensuing reactions (along with energy tally) is presented in the figure below. First the proton and photon collide to form a delta baryon. Then, the delta baryon decays into a proton and a pion. The pion decays into a neutrino and a muon. Since the muon is almost as massive as the pion, all the energy that's left for the neutrino is less than 40 MeV. The muon decays in turn to an electron, a neutrino and an anti neutrino. Since their rest masses are much smaller than the energy involved, each gets approximately a third of the energy. Hence, even though the entire energy involved in this reaction is in excess of 1 GeV, the typical energies each neutrino ends up with is just a few percent of that.
