For a short time I had a 942 that was a refurb replacement for a 921. It had the Providence RI locals burned into the guide (in red, I could not see the programs). Tech support could not get these channels out. They offered me another receiver but I declined. They sent me a 622 anyway (with the HD enabling fee waved) and now I'm a happy camper.
Had there been a local in my market on any of those channel numbers they would not map-down correctly, as the Providence channel numbers took precidence over anything else.
That experience leads me to make the following presumption: A superstation should not map down to it's OTA channel due to the fact that another market may have that same OTA channel and the superstation map-down would get in the way of the local channel mapping-down.