S_QOS_DEFINITION has an identity field qos_def_id which increments sequentially. When this table is truncated, the next value inserted will be last-max-value+1 - e.g , if last max value of qos_def_id is 129, then after truncating, qos_def_id will start from 130. You can check the current number by issuing the following command in SQL Studio:
DBCC CHECKIDENT (s_qos_definition, NORESEED)
You can use this process to reset qos_def_id if needed.
CA UIM QoS
To reset the number so that qos_def_id starts from 1, issue following command:
DBCC CHECKIDENT (s_qos_definition,RESEED,0)