Mark,
You're referring to the Structural Spreadsheet Report and not the Quantification tool, correct? If so, that weight data doesn't come from the Family + Part system but rather from the applicable section XML or MDB files. See the topic "Structural Quantities Property Details" in the delivered Help file - is that what you're looking for?
Edit: This Wiki also includes what I think may be relevant info: Image may be NSFW.
Clik here to view.
