Swap them and see if the same tube is brighter. If so, it is most likely that the filament (which is what glows) is just sticking up and out a little more from the cathode sleeve it is slid into, which is just a manufacturing variation, is common, and not a single thing to worry about.
What will cause my EL-34 filaments to glow differently.
I have a modified Dyna St 70 with a Curcio driver board. My right channel tube filaments glow one brighter than the other. I have checked the filament voltage and it is the same on both sides, left and right. There is nothing in the circuit that I can see that would cause this imbalance. I have moved the tubes around and the condition remains with all combinations.