Modal analysis based on DNV offshore code


I have created the model ran on dynamic analysis.

First, I modelled based on ASME B31.8 code, and the modal analysis results (%Total captured mass) equal to about 80%,

and then I changed the code to DNV offshore, but the modal analysis results (%Total captured mass)  equal to about 40%.

My question is 

1. Why the modal results in two different code are different? (My perception is frequency results depends on stiffness and mass of the system) 

2. I noticed something in dynamic analysis box when I'm using DNV offshore code, pressure stiffening is closed box, is this factor affect to the frequency results?