Pos X and Pos Y are (x,y) coordinate for center of the object.
The default for (x,y) is refer to middle of the object. (You can change [b]Offset[/b] property to change it;s reference point).
You should define x,y and set Pos X, Pos Y properties to x and y.
You are welcomed to check out description for those properties at http://www.um.es/fem/EjsWiki/Main/ElementsShape2D