You are correct. However the real reason you are seeing both channels turned on is because the way Dish has thing set up Discovery HD Theater a la carte for $7.99/mo (at 61.5 and 148) cannot be activated at the same time as the HD package for $9.99/mo (at 110). There were many people that signed up for the HD package only to find that they lost Discovery HD they had been subsctibing to. So Dish unlocked the channel (9420) for everyone with HD receivers last week to prevent chaos!
It's expected that channel 9420 will be locked back up tomorrow afternoon and only visible to those that subscribe to DHD only. By the same token DHD on channel 9421 will not be visible to those with the a la carte channel because 9421 is part of the HD package.
There is a similar thing with the NBA channel. Those with the NBA League Pass subscription who do not have America's everything pack see it on channel 570. Those with AEP and not NBA League Pass see it in the 400 range. Only those who subscribe to BOTH NBA and AEP can see both locations of the same channel.