Hes not trying to build the fastest speed, He's trying for a lower speed, lower recoil.
Using the fastest powder that can generate the desired velocity while staying within pressure limits, will result in less powder used, less recoil, a cleaner gun and usually lower SD.
ANd you are dead wrong about burn rate having no effect. thousands of USPSA shooters would disagree.
I can load 124 grain 9mm's at the same velocity as WWB 115's and my loads generate noticeable less and a flatter recoil. Simply because I use one of the faster powders that can get the job done, While I am sure Winchester, uses a "medium" powder that can work in many rounds due to economy and the fact lots of uninformed people equate recoil with power.