It's a bit hard to explain, and remember, this is all founded on educated guess on what MAY be going on in the set.
In normal operation, there is a fair amount of power flowing between the plate and the cathode of a given tube, this causes the plate to heat up, and this heat is felt on the outside glass, if the tube has no power flowing but still has it's heater on, it would be much cooler.
And of course, if overloaded, MUCH hotter.
https://en.wikipedia.org/wiki/Glowing_plate
So, this could also mean that if there was a biasing problem like a H/K short or leak that would prevent the flow of power between the plate and the cathode, or any grid issue, it stands to reason it could be colder than normal. In the same way that a problem causing excessive flow makes it hot.
In other words, the radiated power from a H/K short has a much harder time making it to the outside of the tube than an overloaded plate.