I suspect that this is because you are buying a FRTW, so you would have higher
priority that just D class for the flights where F/A is not offered.

It may also have some bearing on those flights where there is a surcharge for
some flights on a CRTW, but if you have bought a FRTW the surcharge would not apply.

If you were buying a CRTW, I suspect the booking classes remain the same.