In reply to:

I might also venture that the electrical box and associated connection to the wall provides enough resistance to "trick" the powerbar into a grounded state.

the box is probably anchored to a wooden stud or else just in the drywall. either way, i think it'd be too high a resistance for the powerbar to register it as grounded. of course, it could be a steel framed building, in which case the box might be grounded.

Who's the more foolish, the fool or the fool who follows him?