Jacob had thirteen children. Twelve sons and one daughter. From all the children of Jacob, only one was born from love – Joseph. All the rest were the consequence of contracts, agreements, and a vow.
Love in Hebrew is אהבה Ahavah, Aleph-Heh-Bet-Heh or in numbers 1-5-2-5 which in total has the value of 13. Also, the word one in Hebrew אחד, Echad, which is written within the letters Aleph-Chet-Dalet 1-8-4 has the amount of 13.
Joseph, who born from love, has both the ability to love and the ability to unify. Since Joseph is the one who loves and units all brothers, he does not count as a tribe. For a tribe by definition, has a very narrow outlook at life, like a child.
While Judah, Reuben, Benjamin and the rest of the tribes were coming back to the land of Israel after the slavery in Egypt and inherited the estate, Joseph didn’t inherit the ground because he is the one who is with God. For only in the earth there are tribals distributions, but not in heavens. In heaves, everything is one.