I would think that muzzle blasts are generally consistent within a given load..
Just trying to debunk something that goes against everything I know about reloading, barrel length vs velocity, the barrels used in obtaining velocity figures for powder manufacturers, etc.
I have seen crazier things though...
If I were you, I would move back just a bit and shoot those same loads over the chrono....just for the sake of getting at least a five shot string you can take an average from (where they all register.)
Just my two cents. If you do so please let us know. I am intrigued by this situation!