Behind this is former service as a U.S. senator (only giving the senator seniority within his or her new incoming class), service as U.S. Vice President, a House member, a cabinet secretary, a state governor, and then by their state's population, respectively.
This history encompasses prior service as a U.S. senator (acquiring only seniority within the new incoming class), U.S. Vice Presidency, membership in the House of Representatives, a cabinet position, state governorship, and then by their state's population, respectively.