The difference could be explained by the densities and thermal conductivities used for calculation. For example, in XZT specification, Cr Corundum (铬刚玉砖) only has the thermal conductivities at 1000°C and 1100°C and Insulation Clay (致密粘土砖) has only the upper limits at 400°C and 600°C. So I adopted the upper limit values in my calculation. Clearly this will give us a conservative result, i.e. higher surface temperature.