EPC GaN HEMT - why is reverse conduction threshold different

For reverse conduction in GaN, the same 2DEG is used that is used in forward conduction.
So, why is the reverse conduction threshold voltage (Vsd) for a certain current different to the forward conduction threshold voltage for the same current?

the reverse conduction when the gate is ON is identical to the forward conduction. When the gate is OFF the mechanism is different: you can look at an explanation here
Why do eGaN FETs conduct in the reverse (S-D) direction with about 1.5V? - Characteristics of GaN FETs and ICs - GaN Talk Support Forum EPC (epc-co.com)