I knew when I wrote that, someone would ask.  Since you are measuring the heat output, anything and every thing that effects engine power will effect the temperature. Probe location, cylinder cooling, ambient air temp, altitude, throttle setting, turbo boost if you have it, engine condition, timing, spark plug condition, air humidity, mag condition, impulse spring condition, exhaust system condition, gasoline. airfilter condition, and I'll bet there are some I have missed. Between cylinders the probe location, valve condition, baffling(cooling), and injectors can and do change the indicated temps for each cylinder.