As a setter, you are the first person who must do what the coach tells you to do. If the coach cannot trust you to do exactly what they say, then they will not have you on the court. The setting position is too critical of a position, for the coach not to believe their setter will do what they say to do.
Your coach may have reasons far beyond the court for telling you to set a certain hitter - Maybe that hitter's parents have been chewing on the coach for a couple of weeks because they feel their daughter is not getting enough sets in matches. Maybe the hitter is getting evaluated and the coach saw collegiate coaches on the court, and wanted to showcase that hitter in a certain situation. Maybe the coach wants to see how this certain hitter responds in a pressure situation when the block and defense is stacked against her.
Or, maybe the coach believes the left side hitter is still a better statistical option against two blockers than the right side against one?
What I suggest you consider, is how do you manage your coach?
The bottom line is you need to do exactly what the coach says. Then, during a time out or in between games, let the coach know that you can see the block shifting early towards a certain hitter or let them know you think you can get a one on one situation by going with certain set. Do what the coach says during the game, then communicate your observations during time outs/side changes about what you are seeing on the court.
Coach Matt Sonnichsen