Pressure calculation problem

Hello all

I am working on isotropic 3D system(888) containing ellipsoids(one type only) and using gay berne interaction. For the same temperature and initial conditions but for different pressures, firstly I ran NPT simulations using fix npt/asphere for millions of time steps and calculate the corresponding average density. Then system is made to run NVT simulations using fix nvt/asphere for millions of timesteps and get the pressure. But the output is like, for each averaged pressure, its difference from the actual pressure(from given data) is constant. What can be reason that every value gets increased by the same number?

Thanks in advance.