Something that's helpful is to use your tempgun on a mercury thermometer to see how far it is off. Then you can get the error of the unit pretty accurately.

It might be the case that the tempgun and the helix are off that much and the ranco is on. My ranco's actually run hotter than my probes and thermostats...... (ralph davis's latest youtube video also notices the rancos running hotter for his incubator)

Maybe your unit if off?