Launch Danger Zones are a highly variable result of calculations and also on subjective judgement. For instance, for the first Delta III, evidence was presented that the Delta III was merely a modified Delta II. The Delta II was a well known rocket with very good reliability, being launched by a reliable company. So the Launch Danger Zone was designed like that of a Delta II. When the first Delta III failed on launch, a more restrictive Launch Hazard Area was required. Two of three Delta III rockets failed before the Delta III program was cancelled.
The Federal Government has the responsibility to protect the public for operations from its launch ranges, and for now, also for launches from private spaceports. The current standards on how many people you can kill (30 in a million) or impacts on Turtles, or Whales, and otherwise, are loosely based on the hazard to the public from experience based on aviation risk. This remains a disputed standard in the industry considering that many space launchs, unlike airplanes, are 1st time or early life cycle flights testing new technology. That is why all existing commercial vertical launch spaceports are located directly shoreside and have never been allowed to fly over private property or people, even if they are on a boat 30 miles offshore or 600 miles downrange.
It is likely that the LDZ presented by the County is based on the MINIMUM overflight clearances that may be possible for some undefined rocket.
Note that the County maps show a 4 mile wide Danger Zone surrounding the Launch Pad; the Union Carbide Hazardous Waste Landfill falls within this zone.