Are you saying the restraint loads are different in your W+P1 cases? How different are they?
What happens when you run P1 alone in both programs?
If it's only a difference in calculated pressure stress, the difference may be caused by the equation used to calculate the longitudinal pressure stress - one may be using PD/4t and the other may be using P(A-in)/(A-xs). This is a configuration file setting.
Is this a B31.3 analysis?
As you can see, more details are required.