Frankly, I don't see how a person can call balls and strikes given the definition of the strike zone. The upper and lower limits aren't thin lines. They can't be discerned with precision for any one person, let alone that they differ with each batter. The strike zone is a 3-D invisible space in which a pitched ball only has to partially enter the imaginary space to be a strike. A strike can enter the zone at one point and exit the zone at another point. The umpire is observing at an angle to the plate. The pitch crosses the plate in a fraction of a second. My impression is that the players ask only for consistent calls.