My opinion only:Because your arm feels healthy and you report no pain; I would disregard the radar gun reading and consider it as an inconsistent reading based on your previous knowledge of your usual velocity. I think that the radar gun is used much too often anyway.
Remember that the reading on the gun can vary depending on the make; the setup (by this I mean the angle that the operator is standing at - and the reading will always be less as the angle increases rather than being directly in line with the flight of the ball); and the care and maintenance of the unit.
I used traffic radar for 12 + years. These had to carry a certificate of accuracy and be tested every 60 days to ensure that the speed was correct. I'd be willing to bet that some of the radars being used out there aren't as accurate as one would think or hope.
Had you not been told the radar reading....you'd be pretty satisfied with your current arm condition...right?