Short 10 Velocity Question

[Randy Roberts]

I had loaded up some 1 1/4 oz loads using a published and tested recipe of Longshot, Ched hull/primer, Rem SP10 wad, one 1/4" and one 1/16" cork filler wad and an overshot card. The recipe listed the velocity at something like 1230 FPS. I backed it off a grain or 2 and the chrony had it at 1190 FPS. Perfect!

Then I decided to try an 1 1/8 oz payload with the the same recipe. Other than the shot charge the only things altered were the addition of one more cork 1/16" filler for stack height and the elimination of the overshot card. I expected the velocity to increase but to my surprise it dropped to 1080 FPS. I did not chrony both loads the same day to validate this but both were done under conditions favorable to the chrony. How can this be ???