OK the reason is that you only have one tuner that is capable of doing ether digital or analog but not both at the same time (i.e. duel tuners). When Media Center looks for new tuners it will ask analog or digital ounce you choose one it cant find another tuner to do the other. You will need to have a duel tuner or another card to get both. MC does not have the option to switch between them as your dealt program does. This is why you have more channels on quickplay.
I have the ATI HD-650 and it is the same way and I use cable so I just gave up the Digital Air Broadcast. The ATI Catalyst Media Center is just like yours. But MC is much better so.....
I know this is not what you wanted to hear but this is how it is.
I Hope I covered all your questions but if I didn't let me know.
Later,
DemonSeed
PS
If I helped please hit the scales on the top right and give me some rep Thanks.