Personally, I do not believe that straight edge bit is totally reliable. Many doubles, particularly relatively lightweight ones have their OD's increased in the choke area so they are a bit "Swamped". Also even if proved to be perfectly straight there is still the matter of did they get set to the absolutely correct convergence, which of course can vary a bit according to load used.
It's pretty reliable if you check the other barrel and it is straight. If it is swamped, then of course the other should be, too.
Milt, Just from personal experience, be diligent to hold the gun with the same amount of "grip" when you shoot each barrel. Do everything exactly the same. Guns are much more susceptible to how we grip them, in where they print, than many realize. I found this out with a lightweight double in the past. If I gripped it too tightly it cross fired. If I gripped it normally, it was perfectly regulated. So, if I had gripped it tightly with one barrel and looser with the other barrel, I would have gotten results similar to yours.
I believe that the lighter the gun the more susceptible it is to this variation in grip. Sometimes we get "jacked up" when patterning, and try to grip it too tightly in order to "aim" it closer.
Good luck, SRH
