Matched pair of tubes, but different glow in each.

A friend of mine just recently got some tubes for his amp, and he noticed that when he turned the amp on, one of the tubes "glows" more than the other one. The sound is the same in both speakers, but he would just like to know what causes this ?
I dont believe that the tubes he purchased were new, but were a matched pair.
My Western Electric's do the same--came that way from the factory. They assured me that it was not an issue. I guess just like people, no two tubes contain the same amount of gas.
Couple of possibilities and a comment. The comment - "matched pair" has little real meaning unless you know what was measured and that measurement is important in you application.

Assuming that they are power tubes it is possible that the tube which appears to glow more has some red "glow" on the plates from overheating. If so this tube could be a problem tube on its way out.

If its a small tube or power tube - some times tubes are installed in alternating positions - with the key on the tube facing in on one and out on the next (power tubes) or with the gap in the pins facing in on one and out on the next (mall tubes). The innards of these tubes are reversed and they will not appear the same when you look from just one direction.
Riley804, please describe EXACTLY what you mean by "glows more". Is it an orange glow, violet glow, and where in the tube is it? Different locations? Does one of the tubes glow very bright orange, or "cherry red"? What is the tube type number, and does the amp have a user adjustable bias control?
As stated by Fatparrot, more info would be helpful. If the tubes in question are power output tubes, one might be biased hotter than the other.
Fatparrot and the others that have responded to this, thank you for your information on this.

I will forward this information to him and let him determine what color the "glow" is in the tubes. I will post his answer tomorrow when I have more information to give.

Thanks again,
Riley804 - also please have him note the distribution of the glow in the tubes, as some have asked in their posts above. I'm not an expert, but this is also important in some cases. Best print the entire thread out for him.
I have got a response from my friend and here is what he says :

"The tubes are RCA 6SN7GTB
The amp is a Sun Audio SV-2a3.
The amp is self biasing.
There is mica holding the plates top and bottom. There are two points on top of the top mica and two points on the bottom of the bottom mica that light up, one tube is brighter, it is more orange/yellow and the dull tube is more orange/red in color.
I switched the tubes and the same one stays bright, the sound is balanced and good either way"

Let me know if this is enough information, or do we need more.

Thanks again.
I asked my father why “matched tubes” glow differently. Although it has been quite a while since he has worked on tubes (he designed vacuum tubes during WW II & is 91!!) his response is meaningful.

The kind of tube about which you are speaking would not be expected to include any gas, which would produce typical glows. The glows you are, therefore, referring to must be of the filaments. Their glow depends on their heating or I square R. If the voltage applied to each is the same, the currents will be the same if the filaments have the same resistance.

The filaments would be expected to be of the same material and physical dimensions, which determine their resistance. Thus, I would expect their glow to be reasonably the same. If you interchange the two tubes, do the same tubes glow as they did before? If the one on say the right is always the brighter, or the less bright, I'd say it was a difference in the voltage applied to the two tube filaments and not due to mismatching of the tubes themselves. If the brighter tube is always brighter regardless of to its placement, I would say it is the filament resistance that differ, and that basically the tubes could be mismatched.