Enthalpy Value Reported under Surface Integral
 

I am wondering if anyone knows how does Fluent compute the enthalpy values given in Surface Integral > Temperature > Enthalpy. The values that I got from fluent for a flow through CD nozzle are in the range of -50000 to 40000 J/kg. These are way off than the values given in any steam table. Why are there negative values as well? I could not figure out what is the reference value (the thermal condition that give zero enthalpy) in Fluent to try to shift the results.

Any helps appreciate.