All OTA tuners have to "map" to the local channels. Most boxes give you a scan feature to search the airwaves for what channels are available in your area, just like when you hook your TV up to cable and tell it to scan for channels. When your receiver finds them, it "maps" it into it's guide.
If you have a local Analog channel 6 and it is CBS, when you scan, you'll get something like 6-1, 6-2 for the 2 available digital channels that CBS is broadcasting. The actual frequency may be UHF channel 44, but there is a thing called PSIP which is broadcast as part of the digital signal and it tells your stb how to decode the signal and the mapping is part of it.