If you pattern from different loads/guns etc even if all put the ame percentage of shot in the 30" circle, you will find if you then draw a circle containing half the area (21.2") concentric with the 30" circle they will not all have the same percentage inside that central area.
Another factor other than bore scrub is an ounce of shot will have a column length in an unprotected bore of about .690" in 12ga, .837" in 16ga & .970" in 20ga. Thus the lower pellets in the column have a heavier load to move upon firing. The reduced diameter by use of a shot cup will of course lengthen these columns proportionately.
The column lengths cited were from old Hercules/alliant loading guides. They did not state the size shot or the hardness. Both would have an input on the exact length, but as long as the shot was consistent it would vary proportionately. As I stated earlier I have not counted enough pellet holes to say positively just what effect the actual bore size has on pattern but I do know that not all patterns have the exact same central thickening.
People who concentrate on extreme long range shooting consider central thickening an advantage, most of us consider it a disadvantage & prefer to lessen it as much as is possible.
My personal opinion is this is most apt to occur with the shorter columns of a bigger bore.
