In short quantum stuff. With additional kinds of neutrinos, you get different patterns of oscillation, and the experiment is located at a spot were sterile neut

In short quantum stuff. With additional kinds of neutrinos, you get different patterns of oscillation, and the experiment is located at a spot were sterile neutrinos change the oscillation pattern in such a way that you get additional neutrinos, instead of less as one would expect if they would just have another classical oscillation mode. Perhaps a good analogue is the double slit experiment, the wavefunction interferes with itself and you get more photons at the peaks, not only less at the dark patches of the interference pattern, as one would expect in case of just blocking some of the light.