I'm getting errors in the reporting of mass, when using inserted parts.

I have a part, with other parts inserted into it.  When I use "Mass Properties" to report the mass of the part, including the inserted parts, I get a value.  If I use the "Delete Bodies" command to remove the inserted parts, and then measure the mass, I get a different value.  I expect the two values to differ by the total mass of the inserted parts, but it doesn't.

As an illustration, here is a simple aluminum part that weighs .0377 pound.  It has a diameter .125 inch hole in the center.

Then, here is a diameter .125 inch steel dowel pin, that weighs .0017 pound:

I insert the dowel pin into the first part, and measure the mass.  I expect the mass value to be  .0394 (.0377+.0017), but it is not; it reports .0383 pound.

When I model this as an assembly, it correctly reports the mass as .0394:

Why this anomalous reporting of mass when using inserted parts?