The Unique Client report covers any client that started the connection during the specified time period or ended the connection during the specified time period or connected during the specified time period. The specified time period refers to the reporting period that you specify while scheduling the report.
This is a detailed report that displays the statistical information of the various unique clients.
Check more info here: http://www.cisco.com/c/en/us/td/docs/wireless/prime_infrastructure/1-3/configuration/guide/pi_13_cg/reps.html
Actually, I have already found the configuration guide, I have seen the information that you provided, but what is meaning of "started the connection" ? associated clients, authenticated clients or other kind of clients?