Coupling Constraint Abaqus

I used *Coupling to make sure the one-direction displacements Ui of the nodes on the same plane are the same, but the question is the reference node can not be localized on the plane so that I dont know which reference node to choose near the plane. Additionally, even I choosed a node closed to the plane , the Ui of the nodes on the plane are different, who can explain this problem? Thank you so much!