lhp, initially I agreed with you. Then I realised that takata was actually comparing like with like. He has a series of plots all similarly affected by current and modified by wind driven surface motion.
By taking one common type of object then he was able to deduce the combined drift rate. He parts of the wreckage been included then the plot would not have been as pure. It would be possible to do a similar exercise with debris alone and compare the two plots.