Why do DVD packages which claim the film is in "Dolby Digital 2.0 MONO" house a DVD which plays back the soundtrack, dialogue and everything from the center channel speaker only? Shouldnt the packaging then say "1.0 MONO"? Wouldn't 2.0 soundtracks be considered STEREO, and then converted into surround via Pro Logic II on a receiver?

I have a few DVDs which read "Dolby Digital 2.0 MONO" on them, yet only play from the center channel speaker. Other DVDs say "Dolby Digital 2.0 stereo surround" or some such rhetoric, and they decode in surround automatically through my receiver's Pro Logic II feature. I thought MONO meant one channel, hence 1.0?