aerobat:
i think slippery pete is pretty right with the higher compression- lower exhaust temperatures at pistons . you can see this effect also in tubine engines where maintaining the same power output / fuel flow in a climb will result in exhaust temperature RISING due to thinner air and worsening compression of the compressor stages.
The old correlation is not causation phrase springs to mind. There is another simpler explanation though: in a turbine, air is used to cool the combustion chamber but seeing as you are still using the same amount for combustion you have less left over for cooling as you go higher. Less cooling air passing through combustion chamber means less in the zorst as well.
At least that's what I was taught at groundschool and have always attributed the rise in PTIT to. If there's another explanation I'm all ears.