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.