Perhaps I wasn't clear enough on this point: copyright of a derivative work is distinct (but not inseparable) to the copyright of the original work.

So portions of a derivative work are covered by the original copyright, and other portions may be under a distinct copyright as a derivative work, and several copyrights may apply to a work as a whole.

In the case of a Huffman transform, the transformed work does not meet the "creativity" requirements to be eligible for copyright, over that of the original works.