Why we have difference in allowable for perticular material when doing WRC 107 with CAESARII and PV Elite.

Example: SA-537 CL2 thickness of plate: 40mm.
In Caesar WRC module allowable is: 158 MPa at 80 deg-C.
I have WRC cal from vendor in PV Elite 2012 which has allowable: 229.60 at 80 deg-C.

With all inputs are same i found difference in calculated stresses. Which is more reliable?