mass properties discrepancy in Assemblies

Hi all,

I am attempting to locate the CG of an assembly in SW 2011. It's essentially a box with a lid and 4 mounting brackets- lid and box are AL6061-T6, and brackets are plain carbon steel. The mass and volume of the assembly is about 1.7 times the mass and volume of the six individual parts combined. Any ideas on why this is?

Also, why is density in the "mass properties" window showing 0 for all of the parts? Why does it not list the density for the materials defined for each part? It does show, "grayed out", the correct density value under the "options" window.

Thanks