You're using a laser one right? if not the bettery, I find you gotta try for an average sometimes. Maybe it could be catching some refraction over the distance and brightness, distorting the reading?
The one I have says 1000 yards. At certain times I've had it as close as 908 (perfect conditions, dead nuts still), better results in the 800 range and reliable in the 700 range.
I saw one that was rated for 4500 yards once, but it cost too much.
With that cannon you should be spending up like $2.50 a shot, or more? Like playing golf at pebble beach!