Dart shots are not evenly distributed. You are more likely to get the height wrong then the width - this is because of how the dart travels.

Darts dip a lot in flight - they do not move side to side as much. As a dart player, I guesstimate that after only a few hours practice most people will be off 1.5cm or so in the vertical but only 0.75cm in the horizontal

Professional dart players are always aiming at 20. If they block the triple-twenty with their first or second dart, they then aim at triple-19.with some players preferring triple-18 since height-wise it is in similar range