First of all, thanks to all who responded.

I think cybmole's explanation is the right one. Let me use an example from disc 3, as presented by dvdfab 4 with pathplayer activated when needed (this would be one of those times :

There are 4 chapters on the disc. But you can see here 5 "episode length" possiblities, plus a much longer one. Two of them have exactly the same length.

So I'm guessing there's more than one copy of each. The individual chapters would be titles 5, 4, 3 and 1... in that order (CONFUSING), titles 1 and 2 are the same and title 6 contains titles 5,4 and 3 (times almost exactly add up)

This makes sense, in the context of "nothing about this DVD's structure makes sense so let's try to figure it out". Am I right?

The idea of watching the chapters one by one and taking note of the DVD player's information is time consuming, but I am guessing is the only one "fool proof" way of being sure.