One single itinarary does not constitute a through ticket. They are two different things entirely. Someone can hold a separate paper/eticket with one airline and a separate paper/eticket ticket with another airline for the same journey. The travel agent then takes it upon themselves to type the information onto a sheet of A4 thus producing one single itinarary and incorrectly tell the passenger that they will be through checked
......however both tickets still remain separate and through check-in is not an option!