Most cable systems use frequencies below 54 MHz (VHF TV 2) for communication back to the cable provider from cable modems and digital apparatus, so any "Cable 1" channel needs to avoid operation on the original VHF Channel 1 frequencies from the pre-1948 bandplans.
HRC and IRC systems increase the spacing between channels 4 and 5 to a non-standard 6 MHz, inserting "cable 1" between channels four and five.