According to the bar website:
http://www.smogcheck.ca.gov/stdPAge.asp?Menu=/includes/Menu_GenInfo.htm&Body=/Geninfo/Factsheets/Test_Only_Directed_Vehicles-Aug_2003.htm


Two Percent Randomly Selected - As required by State law, two percent of the vehicles in the Enhanced Areas are also directed to Test-Only stations for their Smog Checks. These vehicles are selected randomly to evaluate the overall effectiveness of the Smog Check Program.