Here is how Brindle lays it out. There is no such thing as a perfectly evenly distributed pattern. Do this as a percentage of the whole and you will see. Burrard must have been making it up. Fine, so now you believe that ALL patterns are hotter in the center than they are at the edges.
Denser patterns are even hotter in the center than looser ones, but they are all center biased.
As you have often read in this space, you want the pattern that will put the most pellets into the effective fringe of the pattern. What we will find out in the following is that for certain target angles this may take more choke than you have been normally using. This fringe ring gets bigger for a while and then it shrinks. This is the magic number for most clay target patterns. This is also just about what full choke is supposed to deliver at 40 yards, modified at 30 and improved cylinder at 20.