I can't remember if you guys use PAL, but I'd wager you do.
You're not missing anything, if that's the case. American DVDs are at 29.97fps, and UK (and I'm guessing Aussie) DVDs are at 25fps.
84 minutes / 25 frames X 29.97 frames = 100.6992 minutes.
It's just the DVD encoding. You don't get that on blu ray or higher, because the technology used is more consistent. It's a hangover from the old analogue TV days.