Do not confuse voltage reading on the TIU, which should be reasonably accurate, with variable channel voltage reading on the remote, which is only accurate (sort of) when the input voltage is 22 volts.
Do note that the Track Voltage reading on the remote softkey is read by the board in the loco, which is downstream from the rollers and the fine wiring in the loco. Between that sensor circuit and the voltmeter in the Z4000, there is also the track, the leads, and the TIU, all of which tend to reduce voltage.
