[Boatanchors] Rectifier tube question

[Yong Surk Lee]

While calibrating a TV-7 I came up to a question that why rectifier tubes like 5Y3 
always show unbalanced test results when tested at TV-7. I tested new five different brand 5Y3s 
on my TV-7 and they were all good and showed 4 - 6 higher at JS0-6000 than JS0-4000. 
I guess this means two diodes in 5Y3 are unbalanced. 
Also 83 has different test data SHUNT values between plates. 
Could someone explain why? 
Thank you in advance.