The confusion arises from the fact that "Dolby Stereo" in cinemas is actually equivalent to "Dolby Surround" in the home. There would typically be 3 speakers across the front: Front Left, Centre and Front Right, then several surround speakers placed down the side walls and across the back.
The mono mix I believe was created for smaller cinemas which didn't have Dolby Stereo installations, and for TV broadcast.
There are some great resources at Dolby's Website that explain all this.