Well, a thermometer (embedded in your thermostat) should read identically in the same situation. I've got radiant and use forced hot air as a backup - the forced hot air does get the air warmer quicker, but the radiant is more comfortable. Except when coming back from an extended time where the house has been turned down, I use the radiant and keep the forced air thermostat turned down a few degrees below when the radiant is. The Wirsbo thermostats deal with a much bigger heat load...they are trying to heat the structure rather than just the air, so when they turn on and off verses a thermostat designed for hot air could be different. For example, the radiant might turn off before it actually gets to the set temp, thinking that the mass will continue to heat up a little as it migrates from the burried pipes to the room. This would prevent overheating the house.
Shorter answer, I don't know. I'm surprised they show that different of a reading.