Hi,
Chuck replied:

Tube testers are a compromise. The manufacturers of the testers try
out a lot of different tubes to create their tube databases. The Gm
values bear no resemblance to the values found in the tube manuals.

That pretty well sums it up. Incidentally the 3444 is a really
good tester, that runs tubes closer to tube-manual conditions. But as
noted, any Gm tester will rank tubes in about the same order, good to
bad.

I normally use a TV-7B and a KS15874 Cardmatic.

73, Alan