I don't know much about this off the top of my head, but roughly, the idea is that a neutrino we observe with a definite mass is a mixture of the three flavors,

I don't know much about this off the top of my head, but roughly, the idea is that a neutrino we observe with a definite mass is a mixture of the three flavors, and a neutrino with a definite lepton flavor is a mixture of those with different masses. The different masses mean that as a neutrino of a certain flavor travels its different constituent states evolve at different speeds, so what it is measured at have different chances of being measured based on when the measurement takes place. Photons have no effect like this because they are not mixtures of different particles and have no concept of 'flavor'. Massive photons would be the same.