if you look at the pics the type of socket is slightly different, this is the only reason they aren't interchangeable

This is a thermocouple. Don't get too tied up with the accuracy of this, they can only be used for a comparison reference. The problem is that the cooling fan blows across the cylinder head which in practice always cools the thermocouple faster than the heat conducts from the spark plug seat so the actual values have to be either calibrated for this effect (impossible really) or ignored and the reading is just used to help show anything abnormal rather than real values.