Icing conditions won't appear unless the temperature is lower than 8°C so, the only possible explanation is that, those weather engines displayed +12/+13, but the real temperature in the sim was lower, maybe for a brief amount of time.
GSX takes the weather information in the standard way, using Simconnect directly and asking for the Metar so, the only possible reason for this, is the temperature in the Metar for that airport at that time WAS lower than 8°C in the sim, regardless what the other weather engine was indicating.