Scale. In non-clan systems, strong familial bonds beyond one's first cousins are typically weak, and indeed relationships with one's thirds cousins or further are typically nonexistent.
Clans OTOH... well they cover pretty much the entire family tree. So their would be a familial bond and loyalty with ones 20th cousin say. The bonds are strengthened by how relationships in tribal societies are typically inter-tribal, and often consanguineous... so the "family tree" looks a little more like a lattice then a tree