The Height of model I picked is not right?

the pictures shows that I pick two positions on the top face of the model(the face is level and horizental),but I get two different HeightString,anybody knows it?any help will be appreciated!!

My Codes is the sample code published on the the picking sample):

var pickedObject = scene.pick(leftclick.position);
var cartesian = viewer.scene.pickPosition(leftclick.position);

                        var cartographic = Cesium.Cartographic.fromCartesian(cartesian);
                        var longitudeString = Cesium.Math.toDegrees(cartographic.longitude);
                        var latitudeString = Cesium.Math.toDegrees(cartographic.latitude);
                        var heightString = cartographic.height;