There has been a lot of back and forth, and it is somewhat hard to follow, but I know what the TC is talking about. When it is said that the dejure duchy doesn't change, it seemingly appears to when you get a claim on a "duchy" and the counties of that duchy are all over the place. At that point, why aren't all of those territories considered dejure if you can claim full rights to separated territories that are obviously not a block of dejure land?
It's hard to explain, but it's not overly complicated.
The de jure dutchy does not change, even if you have managed to claim for all that dukes lands.
Example: We'll go with The duke of Apulia, in Sicily, and for examples sake, we'll say all he has is that one duke title, and he personally owns all three counties in apulia, that is all the lands he personally owns but he has control over the two counties that make salerno and the two counties that make capua, obviously, they are ruler by other counts.
So, you press your claim for the dutchy of apulia and you gain all of the lands listed above, and the counts become your vassals, you are de facto ruler of all of this. However, shortly after this, a muslim Jihad attacks you and takes it all away from you, you still retain your claim for the dutchy of Apulia but none of the lands.
We skip a generation and there has been a crusade for the whole of Sicily, and the king there is now an independant ruler and has assigned all of the dutchies of Sicily to individual dukes, you decide now is a good time to press your claim for Apulia and you win. This time, however, you only gain the three counties in Apulia and the title of duke of apulia.
You did not gain the lands you had previously controlled as you had no real claim on them, you were the de facto ruler, not de jure.