The neutrino starts in a particular flavor state, ideally purely one flavor.

As it propagates, the mass states evolve at different rates.

After some distance and hence time, the neutrino is in a different mix of flavor states, and the interaction samples that.

If that’s all there is, that final mix of states would always be exactly the sample.

But in real experiments, it’s a bit more complicated. Neutrinos are created with a range of energies, so the time evolution to a given distance is not always the same. Matter effects can add to the mixing. And real beams are not always of a pure flavor to start with.

So the final superposition of flavor states is not always the same in a real experiment.

The neutrino starts in a particular flavor state, ideally purely one flavor.

As it propagates, the mass states evolve at different rates.

After some distance and hence time, the neutrino is in a different mix of flavor states, and the interaction samples that.

If that’s all there is, that final mix of states would always be exactly the same.

But in real experiments, it’s a bit more complicated. Neutrinos are created with a range of energies, so the time evolution to a given distance is not always the same. Matter effects can add to the mixing. And real beams are not always of a pure flavor to start with.

So the final superposition of flavor states is not always the same in a real experiment.