Voltage measurements are affected by length of cable, number of connectors etc. Each contact causes mV range drop. But a briefly check, they are statistically matched.
Voltage conversion factor:
Watts Up: 1.0007xWattsUp reading (V)
EagleTree: 1.0054xEagleTree reading (V)
ICE2 80HV: 1.0061xICE2 reading (V)

Not surprise..... WatssUp is closer to the battery, and ICE2 is at the far end; the reading is lower than the voltage at battery side.
I believe (not try yet), if I series them (to the battery) one each time to minimize the cable length and # of contactors, the voltage reading should be the same

Luke