My guess is the 660lb comes from the limitations of the brunk lid. After all its just plastic.I'm looking at the manual in hope of finding some open statements that contradict the 660 cargo limit but the manual clearly says that on page 6-75.
This is along the same lines as the bed cover holds 220lbs (or something like that), followed by the statement in the manual to not put anything on top of the cover. Clear as mud... I blame lawyers.