This effect is because of the interference of the light sources are constructed at this particular wavelength (around 1.45 microns in your case, I got around 1.32 microns in my test simulation). If you plot the source power in regarding to the wavelength, you will get something very similar to the following plot (with the dominant peak at ~1.45 microns):
f = getresult("monitor", "f");
sp = sourcepower(f);
which indicates that, the source power at this particular wavelength is way higher than at all other wavelengths, so the output E-filed magnitude at this particular wavelength is also way higher for all the branches.
On the other hand, the transmission is normalized to the source power, so you get the reasonable result for the transmission.
I hope this explains it