simple tester uses 1V bias voltage, low Uanode and small AC signal on the grid to "measure" the transconductance.
Take a look on the curve of the tube. The transconductance is the slope of this curve.
As you can see, it's very nonlinear, and the maximum is at low Ug.
This curves are also very dependant to applied Voltages (Ua, Uscreen)
So your tester measurement could be correct, and the TT measurement as well, using higher Voltages and different bias points -which is much more practice-relevant
