Question on NOS tubes bias doubt

I just put a New pair of NOS KEN RAD VT-231 in my CARY SLP-05 pre buffer stage, one for left and one for right, it sounds bias to the left for around say 10-15%, so I turn it off and swap them to confirm if it is not my room acoustic nature, as expected, it now bias to right for same amount. My question is, is it so called BIAS and that is why most tube gear has bias adjustment function? And should this be happening when seller says it is a matched pair NOS? I can compensate it with the balance feature on Cary SLP-05, but I doubt this pair is not matched. The testing figure in fact being declared slightly different while most of them are.
I don't understand your question but that's a real nice sounding tube.
The Ken-Rads are some of the best VT-231's out there, but like anything else: They're not always perfect. You bought a pair of unmatched tubes. A slight mismatch shouldn't be noticable. That you have to compensate with a balance control says they're out quite a bit. Did the seller offer any satisfaction guarantee? BTW: Tube "biasing" is another story altogether(not related to your issue).
If you are saying that the center image of your stereo systems drifts substantially with these two tubes that they must not be matched, despite what you have been told, I agree.

In fact I would suggest that not only were they not matched, but that the weak tube is close to dying. IME, other than an onset of noise or dullness, drifting is the hallmark of a tube which is about to die. IME, perhaps there are other reasons of which I'm not aware. I do not use this tube type.
Both Rodman9999 and Newbee understand exactly what my problem correctly, yes the image shift to one side 10-15%. Thanks for answering all my question precisely and confirmed. Now I have faith to speak with the seller and he sounds pretty responsible, I will post what the outcome is for future case reference to other members.
The tubes could be matched for triode balance and trans-conductance but not for gain. In this case, one tube appears to have higher gain than the other. Just use the balance trim to center the image and enjoy the tubes. That is why its there. On my Cary SLP-2002, I have to adjust the filament voltage on the circuit board with a meter, fidling with a small blue pot. Be glad the SLP-05 has the convenience of just turning a knob on the faceplate.
"Transconductance" IS how you measure the "gain" of an electronic device(tube, op-amp or transistor). You'll note that it is determined by finding the difference between the voltage at the input and the current at the output of the device in question: ( Tube pairs with matched transconductance are sought out to avoid system imbalances. If the tubes in question were matched as to triode balance and to one another: there should be no imbalance from channel to channel as a result of installing them. That's providing there is no imbalance elsewhere in the system that the installation of properly matched tubes happened to reveal.
Thanks for rodman99999 info, I further confirm one of the pair has problem because that one started producing noise after couple of days trial.
The tube becoming noisy that quickly also confirms Mr N's response regarding it's being about to expire.
True, Newbie concluded that precisely. Now that the seller tell me to send back to him for refund or replacement. Thanks all.
Hopefully your next pair will be in better condition. Happy listening Mr G!