Seconds_Behind_Master is always 0 when running show slave status

               Slave_IO_State: Waiting for master to send event

                  Master_Host: 172.x.xx.xxx

                  Master_User: replicate

                  Master_Port: 3306

                Connect_Retry: 60

              Master_Log_File: mysql-bin.000239

          Read_Master_Log_Pos: 51505421

               Relay_Log_File: relay-bin.000002

                Relay_Log_Pos: 670901269

        Relay_Master_Log_File: mysql-bin.000237

             Slave_IO_Running: Yes

            Slave_SQL_Running: Yes

              Replicate_Do_DB: 

          Replicate_Ignore_DB: 

           Replicate_Do_Table: 

       Replicate_Ignore_Table: 

      Replicate_Wild_Do_Table: 

  Replicate_Wild_Ignore_Table: 

                   Last_Errno: 0

                   Last_Error: 

                 Skip_Counter: 0

          Exec_Master_Log_Pos: 763661241

              Relay_Log_Space: 2206074748

              Until_Condition: None

               Until_Log_File: 

                Until_Log_Pos: 0

           Master_SSL_Allowed: No

           Master_SSL_CA_File: 

           Master_SSL_CA_Path: 

              Master_SSL_Cert: 

            Master_SSL_Cipher: 

               Master_SSL_Key: 

        Seconds_Behind_Master: 0

Master_SSL_Verify_Server_Cert: No

                Last_IO_Errno: 0

                Last_IO_Error: 

               Last_SQL_Errno: 0

               Last_SQL_Error: 

  Replicate_Ignore_Server_Ids: 

             Master_Server_Id: 3

               Master_SSL_Crl: 

           Master_SSL_Crlpath: 

                   Using_Gtid: No

                  Gtid_IO_Pos: 

      Replicate_Do_Domain_Ids: 

  Replicate_Ignore_Domain_Ids: 

                Parallel_Mode: optimistic

                    SQL_Delay: 0

          SQL_Remaining_Delay: NULL

      Slave_SQL_Running_State: apply log event