SPH Post-Pro

I have a problem in calculating classic CFD properties starting from an initial point cloud, that is the output of my simulation. I’m now trying to calculate volume of a fluid region.
I used the filter SPH Volume Interpolator, then the Integrate Variables and I switched the Attribute to Cell Data, so I found the volume of the box I used for the Volume Interpolation, but I have a more complex shape and so the volume calculated by ParaView is larger than the real one.

Thank you for your help