msycamore said:

I think it's the name "Dolby Stereo" that confuse you, an optical 35mm Dolby Stereo track isn't just two channels, it's a four channel configuration; Left-Center-Right-Surround.

That's the information I was missing. I understood the matrixing, but assumed that the source had two channels, not four. All is clear now.

Moth3r said:Early 70mm 6-track mixes were actually 4.2 - 3 front, mono surround and 2 LFE channels. You can't encode 2 LFE channels into AC3 hence it becomes 4.1.

That makes sense. Thank you both for the information. :-)