Be heartened by putting the matter in proper perspective, Philly. It has nothing to do with any shortcoming on your part. and likely isn't even substantially improvable for us. By "us" I mean that all non-pros have that same limitation due to the very definition of the word "professional".
-- This is John's profession and has been for many years. That radically shapes the mental elements -- and "focus" is only one of the particular mental elements -- that any top pro has honed for survival in a millimeter-precise profession, and what that contingent of players absolutely must bring to the table and we simply don't have to . . . because we survive and prosper in life by other means and our subconscious knows this.
Consistent, trained focus, resilience in the face of intra-game reversals (challenges to a player's stamina and confidence) these are mental elements that have to consistently exist in the minds of any athletes in any sport, to a degree that most of us would be uncomfortable with, much less capable of consistently maintaining during rack-after-rack of relentlessly precise execution.
Arnaldo