Just to be clear - is it that socket (V8) that measures lower temp regardless of tube arrangement, or a particular tube that the lower temp follows around? Does the tube appear different while in operation, as far as what is glowing and how brightly (assuming you have a matched set)?
If it follows the tube, it's probably the tube. If not, more likely the socket - or some inherent temperature difference/measurement inaccuracy peculiar to that tube slot. You always have to suspect systematic effects on measurement.
Position V8 is at the very front, so no adjacent power tube on one side. It's worth noting that the power transformer is at the back, and that's the one that does get toasty - both output xformers stay cool in operation. So V8 is as far as it gets from the output xformer. I'm not sure where the power resistors are under the hood on this version, but the chassis may not heat entirely evenly. Depending on how air flow works in your measurement position, that could also easily play a factor. Any of these could be playing a role.