This might not be the answer you're looking for. But I do know that some stations will transmit on another RF frequency as a backup or to fill in bad reception areas. I believe the VHF stations do that more than UHF stations. A local apartment building owner uses a MATV system.
I look at the antenna on his roof I noticed it didn't have VHF. I asked him how he gets RF12, one of our local CBS stations. He says he gets it off of another UHF sub Channel.
So, they might do it for apartment owners or people who simply don't have a VHF antenna. Now that I've said that, hopefully you'll get the real answer!
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