A question about GCMC+NVT simulation

Hello all,

GCMC is used to get a supercritical methane system of which the temperature is 360K, the pressure is 390 atm, density is 0.178g/ml.
But when I output the configuration of the last GCMC step and then use this configuration to NVT, I get a different result, 360 K, 344atm, 0.178g/mol, which is match better with NIST data, 360 K, 335atm, 0.178g/mol.
I wonder if the GCMC method is not well in high-density conditions?