Board Thread:Theories and Speculation/@comment-28939725-20161201164545/@comment-25122992-20161206165404

He was called a Senju descendand only once, when Madara mentioned that Nagato was quite suitable person for the Rinnegan transplantation due to his Senju lineage. Even if I just cannot recall when he was called a Senju again, it could still mean that Madara called him that way because of Uzumaki's relation with Senju. It's the most possible version.