Those testers are 60 plus years old so I'm not expecting true accuracy. I want to test for shorts first and foremost. That's what will damage your gear.
Tomcy6, you are dead on. No one tracked the tubes that were in those caddies. Repairmen had tons of tubes on hand. It was common practice for a tech to replace all tubes , that way much less chance of a return call. What happened to the tubes that were pulled? They went back in someones caddy and the bad tube was tossed.
My tester is within 50 microhmos of a 6L6 and 12ax7 that were tested on an Amplitrex so that's good enough for me.