gluUnProject & Matrix

I just study OpenGL.
In my program near plane is 1.8f
far plane is 7.0f
I draw a rectangle in z = -0.2f;
and use gluUnProject to read the mouse click.
But I click the vertex of rectangle vertex,
gluUnProject tell me it is (1.36, 1.0, 0.2f),
actually it should be (0.6, 1.0, 0.2f).
why ?