Stephen, your correct.... you don't know how Creo is making this light grahpics.
Yep had problems with Simplified reps & windchill check-out.
You need to set in config.pro for Windchill
mp_calc_level assembly_only
mass_prop_update_force_change no
save_objects changed
If you solidify the envelope-part (default=surfaces), then the weight is correct.
Or give the envelope the correct weight, in the mass-prop window.