From what I understand Directv generally locates their LRF (local receive facility) for a DMA at a broadcast facility if possible. For example, in my DMA it is located at the broadcast facility for the local CBS & Fox stations (the studio, not the antenna site) So they are able to get a direct feed for the CBS & Fox stations, and use an antenna at that site to pick up everything else. They must be getting the same thing that is being broadcast (i.e. including all the subchannels) because the quality is basically the same as OTA. If they were getting a full bandwidth version of the main station, Directv should have superior PQ compared to OTA.
Supposedly some stations do give Directv that full bandwidth version, but if so I have no idea why some do and some don't - probably it is just up to the station and what they do depends on what's easier for the station engineer. Getting the full bandwidth version could be more expensive for Directv, since they'd need an encoder located at the LRF, so maybe they don't even want that unless the station leaves them no choice.