Difference in outlet geometry and surface parameter

I have 8 inch outlets to approximate pump inlets.  The calculated area is 50.27 squ. inches.  When I put in surface parameters in the results section, the area returned is 47.7 sq. inches.  This is consistent across mu.ltiple analyses.  It does vary with mesh.  Is the value in the surface parameters the effective area the computer "sees", including meshing effects?

SolidworksFlow Simulation