Becton-Dickson, a company that prepares media, reduced the c…

Becton-Dickson, a company that prepares media, reduced the concentration of nalidixic acid in their Columbia CNA media from 15 mg/mL to 10 mg/mL in order to increase the recovery of the desired organisms from clinical samples.  This change does not decrease inhibition of the undesirables.  Why would this change be beneficial for the recovery of the desired organisms?