Not sure how that bias probe works, also not sure if you have installed 1-Ohm cathode resistors for bias measurements . . . but if so, it's possible that the bias probe has its own internal 1-Ohm resistor and you could be reading the voltage across both that resistor and your own 1-Ohm resistor. That would give double the voltage, and thus the tube would appear to have twice the cathode current that you would expect.