What explains the membership of Shingon's 13 Buddhas list?

The list of 13 Buddhas (which is actually Buddhas, Bodhisattvas, and Wisdom Kings) plays a role in Shingon gongyo and funerary rites. What accounts for why these particular 13 are set aside for special attention? - Fudo (Acala) - Shaka (Sakyamuni) - Monju (Manjushri) - Fugen (Samantabhadra) - Jizo (Ksitigarbha) - Miroku (Maitreya) - Yakushi (Bhaisajyaguru) - Kannon (Avalokitesvara) - Seishi (Mahasthamaprapta) - Amida (Amitabha) - Ashuku (Akshobhya) - Dainichi (Vairocana) - Kokuzo (Akasagarbha) Minor update: I did some googling on this, it appears that lists of Buddhas, sometimes very long lists of Buddhas, occur throughout the Mahayana cannon .