I'm assuming it works like the H20, and sees the DSWM as an old school SWM. The way the SWM protocol works the receiver asks for a particular frequency, and the SWM either accepts that or not. Since the H20 (and D12) only know about the 8 original frequencies, that's all they can ask for. So they aren't grabbing the first eight, they're grabbing the original 8, and not the new frequencies that are in between the existing ones - only newer stuff that knows about the DSWM can ask for those.
If you mixed and matched D12s and H25s unless you can somehow guarantee the D12s will start up first and grab the channels they want before the H25s can, some of the D12s may be SOL.
I always thought that the newer hardware should request the new channels first, to better work with the older stuff that doesn't know about DSWM but Directv never listens to my good ideas