Craig, the fact that the proper channel steering depends on proper phasing of the rear channels indicates that the rear signal is not that of a discrete channel. It is similar to the use of a DPL decoder in 3-channel mode in the rear channels to extract a rear-center.

To answer your question, it's not coincidence; it's doing exactly what it should be doing. I have the setup I described, with a DPL decoder, and my playing with the THX tools on my Star Wars, Episode 1 disc did exactly what yours did: produce a rear-center sound separate from the side surrounds.

Larry