From the Flight Global article, it appears the GE and PW design recirculates some of the heated fuel back to the upstream face of the FOHE, thus preheating the new cold fuel approaching the upstream FOHE face (thus melting any water ice before reaching the FOHE). The Trent FOHE does not appear to have this warmed fuel recirculation feature, thus the fuel reaching the upstream face of the FOHE is colder than on the GE or PW designs.