A pitched ball reduces speed roughly 8-10 mph as it leaves the hand. Various guns capture the speed at various points along the way to the catcher.
Better guns capture the speed closer to the hand. A Bushnell gun picks up later and will give a 3 mph slower reading than a JUGS or Stalker.
Some guns give fastest (out of the hand) and slowest (near catcher, but well in front of the mitt) readings. The "peak" (fastest) reading is all anyone cares about. To me, I would rather have a gun that shows both at the same time (so you can identify the following problem).
You may occasionally get different readings on the same speed pitch. The gun may pick up 5 feet out of the hand on one pitch, and 8 feet on the next pitch, and you will never know it except for the slower reading.
Finally, if you stand at an angle to the ball path, the reading will be slower. (So, for fun, if another team's Dad comes over and asks how fast his son throws, just scoot over a few feet and show him the slowest reading
)