Recently, it has been generally accepted that it was called 'mishima' because the irregular patterns were similar to the Mishima-goyomi calendar made by the Kawai family and distributed from Mishima-taisha Shrine.
地紋の斑になったところが三嶋大社発行の三島暦に似ることからと「三島」と称するというのが近世以来の通説。