I always measure mine at the glowplug.....point the temp-gun down the cyl-head hole. The glow-plug temp is pretty constant summer or winter (with the same tune), but the fin temp can really drop on a cold day, or climb on a hot day, so it's not consistent with identical tunes.

I find it odd you got higher temps at the fins than the glow-plug. I normally would get the reverse of your readings.