It's NOT because of memory....
The 6000 has an OTA tuner built in. (yes, its optional) so you get your locals on the 'lower" numbers if you use the tuner.
It will be the same on the 811 & 921 - satellite locals in the 8000's, OTA on actual number.
When I inquired with DishNetwork some time back, that is what they told me.
OTA channels from the 8VSB module all begin with "0" such as 007-0 for Analog 7 and 007-1 for Digital 7. They could still channel map channel 7 for satellite 7 if there wasn't a problem, like memory. There has to be a way to differentiate satellite from OTA, maybe that's the way they are doing it.