Think of prima facie speed as "safe speed for conditions', it is entirely possible to be driving the posted speed limit and get a speeding ticket..
Example of a prima facie speeding ticket; you are driving 37 MPH, the posted speed limit is 40 MPH.... but it's raining heavily, so what's the legal speed limit in this case?
Answer; whatever speed limit the officer thinks is safe... so on your cite will be:
Posted speed: 40
Veh speed: 37
PF Speed: 20 <-- this is the officers opinion as to what the safe speed for conditions is.
I wouldn't get too excited about arguing a PF speeding ticket unless the difference in the two is either big or small, or you can argue the officers opinion was totally wrong.
In the above example, say you bring in a weather report from that day that says it was only drizzling, you can argue 20 MPH is much too slow for only drizzle... then the judge can agree, but decide that 30MPH was the safe speed and still find you guilty... but, usually, with a lower fine.
PF speed limits are totally open to opinion and traffic court judges, in my experience, usually rely on what the 'highly trained' officer has to say.