It’s relatively common in LA and SFO and the explanation is that the girls are being promoted through several independent bookers at the same time.

I’m not sure if that’s what’s happening here in Nova because the multiple websites all seem to point to one singular phone number. I know that doesn’t answer your question because we still don’t know why these agencies in question are using multiple websites.

If so I would change your question to why do Chinese Agencies have multiple sites for the same area/location/escorts.  Most are real in the sense you can book the girl from the info on the particular site you're looking at (sometimes they have the same number, sometimes different numbers).  

 
Most of the Thai girls seem to use an ad site. The Korean Agencies don't have multiple site -- some have changed their name over the years and some have had the same name here for 15 years or more.

Web sites or incalls?