Since ENSO is the result of a standing wave dipole and that the SOI measure takes the difference between two SW extremes, then we should see something like \\( G(t) = C_1 sin(AF(t)+B_1) + C_2 sin(-AF(t)+B_2) \\) as the best fit. And that's exactly what happens. The peak locations are not perfectly aligned to extrema so the fit guesses on how it is partitioned. This can be observed as the iteration process converges.