TreviTyger
Take a copyrighted literary work off the shelf. Read said literary work. Add various paragraphs to various pages of the work, change words within the work, add sentances to various areas of the work. Then, make a catalogue of their changes: the difference between the original work and "their version" their now-modified version of the original work.
They can then distribute this catalogue of changes without any regard to the copyright of the original work.
This might seem intuitive enough but as I mentioned, this is 'cognitive bias' rather than actual copyright law.
In reality, to do what you mention in such an example is illegal.
Derivative works can indeed be separate works with their own separate copyright but only if the original author "specifically" allows it. [Emphasis added]
In terms of "specifics" this relates to an aspect of law sometimes called the principle of specification. What this means in practice is that a "specific agreement" is required and due to the fact it requires "specific permissions" then vague licensing agreements (such as open source) are simply not specific enough to be considered valid by a copyright law judge.
Instead, what would be better is a specific license written up by competent lawyers that outlines in very specific language exactly what derivative works can be created and for what uses.
So here is a simplified example of how specific licensing might work,
A novelist can authorize their book to be translated in to one language to be sold in specific territories by one publisher as a hard back book, and then make another license agreement with another publisher for a soft back edition to be sold in airports.
Then the first publisher would need a separate license from the author to make a softback edition themselves even though they already have a license to make a hard back copy. That's how specific things need to be!
Therefore, one can easily see how open source licensing, when it comes to derivative works, simply lacks the 'specific' language required for a judge to determine how to address such things in terms of actual copyright law.
To put it another way, if vague licensing agreements were adequate rather than 'specific agreements' then you could end up with massive amounts of copyright overreach from all sectors that rely on copyright law to keep things from getting chaotic. Thus the whole system would break down.