I can’t help you with the “why”, but I can tell you where - see attached file (just join the four small surfaces to the main part). The 4 small slivers on the inside are - for some reason - not trimming well. I suspect that it has to do with the seams of the untrimmed cylinders being right on the seams of the adjacents faces. What I did was to untrim the surfaces, use SrfSeam to move the seams “out of the way”, retrim and rejoin. Rhino should of course flag this as bad! item_0041124_Angle_Bracket_8_40x40_Zn_simplified_JN.3dm (511.6 KB)
There are also a bunch of out of tolerance edges in there, if you Explode and RebuildEdges, then try to re-join you will see them. The corner spherical octants are particularly bad, with edge tolerances reporting this:
This is likely also the cause of the 3 inner trimmed cylinder surfaces not being able to have their area calculated and therefore the volume calculation failing. As Jakob said re-trimming them fixes them. But there are a few other areas to look at to get a good closed solid with all component surfaces within edge tolerance.
I have a model that is a closed polysurface that also fails to calculate the volume. I exploded it, ran RebuildEdges and re-joined the surfaces. Not sure if this is going to matter, but this was imported from a vendor, so I didn’t make the original model.