Analog VHF signals may be duplicated in digital format by a cable company in order to combat ingress and deliver a better picture. When a cable-ready set is tuned to a VHF broadcast station that's carried "on channel," i.e. 3 is on cable 3, 5 is on cable 5, etc, there's the possibility that the tv set will receive a broadcast station via the cable signal and via the airwaves at the same time due to a loose fitting, poor shielding in the set or other reasons. When this happens, the same picture arrives at the tv set twice, separated by a few microseconds or so. This causes a ghost in the image. By offering the VHF broadcast channels in digital format for STBs and cablecard-equipped sets, the ingress problem is neatly solved.
The UHF broadcasters aren't carried "on-channel," since cable channel 55 is an entirely different frequency than UHF channel 55, and it's the same for the other UHF stations, so there's no ingress issue to work around.
Now the way that a cable company transports the broadcast channel between the headend and the neighborhood can be analog, digital or some combination of both methods depending on the requirements of the system.