Why is it that for some digital channels, I need to tune to a UHF channel number such

[SolidSignal_Cliff]

The phenomenon you're observing is related to PSIP channel mapping. PSIP stands for Program and System Information Protocol (PSIP) channel information. PSIP is data that is transmitted along with a station's DTV signal that tells DTV receivers important information about the station and what is being broadcast (what channel to tune to for reception of the station, helps maintain current (analog) channel number branding, tells the receiver whether multiple programs are being broadcast, etc.).

ref: AntennaWeb