I'm pretty familiar with the physics behind the technology. I was an electrical engineer for a decade. The fact that the signal travels at the speed of light has nothing to do with how well it measures speed. The measurement is a function of the power emitted, quality of the sensor and the design of the electronics. It's like saying all speakers are the same because sound travels at a certain speed.
My take on it is that the power emitted is lower and maybe the sensor is not high quality so it needs the object to be closer to get a good reading. Or I could use economics to explain it. IF a $100 radar gun was as good as a $500 gun, then there would be no more $500 guns around.
I think this is accurate. I have the Slugger unit I posted this thread about on demo. I used the unit 5 feet in front of home plate and then moved it to about 15 feet from release. The measurements were consistent. When the unit was back readings from 47-50. When the unit was closer readings from 50-53.