It really depends on how they are being used in the circuit and how each one has aged.
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The way the filament is setting inside may have something to do with it. If you use a regular bread toaster, look at the heating element inside it while it's toasting. The element will look dimmer if it's leaning a against the insulator. The one tube filament may be touching the cathode more where it looks dim. I'm not saying the tube is good, but all kinds of identical tubes do have a different brightness in the filaments although it's still putting out the same heat.