Actually, I was speaking of my A/V receiver, an Onkyo TX-SR700. It may have just been 3/1 (no LFE), but I know I've seen it display that there was only one rear channel. I don't find this totally surprising, as there were many movies that were shot with a matrixed four channel soundtrack (L, C, R, & rear). In fact, I think I have a couple of DVDs encoded this way (the remastered Journey to the Center of the Earth is one, I believe).
My point being, that there was DEFINITELY no stereo effect in the rears.... ALL of the rear content was going to the center rear. I know from past experience with my receiver that when there is a mono rear soundtrack (like 3/1), it does NOT put that mono track into the center rear channel (even though I keep it in EX mode). Instead, if the receiver knows it's getting a mono rear track (like 3/1), it places that mono track into both the L & R rear channels. In the case of the show I was watching last night, it was apparent that mono sound was being piped over the stereo rear channels, causing it all to end up in the center rear channel (via EX decoding). In properly mastered material, if the rear channel is mono, it is encoded as such, and the receiver KNOWS it's a mono channel (and therefore only displays "1" rear channel).