The tube testers in the stores were probably calibrated to fail slightly weak tubes
I am pretty sure they were all "calibrated" to sell vacuum tubes.
After I spent big bux to buy a Magnavox color TV in the late 1960s, touted as "fine furniture" by the snake-oil salesman in the Rike's department store, the first thing I did was to purchase a complete set of replacement tubes, except for the shadow-mask color CRT. Then, whenever the set quit working, I would replace what I thought was the most likely defective tube. I got pretty good at this, but after almost thirty years of service I finally had to "let it go" in favor of a couple of Samsung 1024p flat-screen TVs purchased in the 1990s. Got ripped off again on the price, but at least the picture was good. Sound, not so much.
And hams and bombers DO care about distortion. Hams because distortion causes frequency splatter from their linear amplifiers. Bombers because those tubes were used in wide-bandwidth distributed amplifiers, very similar to the distributed amplifiers used in the vertical gain stages of Tektronix oscilloscopes, except gigahertz bandwidths and obscene power levels. Enough power to shut down TV reception for hundreds of miles around the bomber, which usually meant shutting down TV reception across a lot of Canada, since there was little else near Kincheloe AFB, MI.
