Object width in treadmill script

For a treadmill effect, I have an array of objects that I shuffle around. As the objects in back go off screen I move them to the front. The way I calculate where to place objects in front is as seen below. All these objects are simple boxes of varying sizes with box colliders. For some reason, 2 of the 10 objects that I loop thru have approx 5 unity meters of padding between them and subsequently stacked objects. The only difference I can tell between these 2 objects and the others is that these 2 are unity cubes and the others are meshes imported via fbx. Can anyone explain this mystery space? Something must be wrong frontObject.transform.position.z or frontObject.renderer.bounds.size.z values.

function Update () {

     var frontObject: GameObject = slabs[slabs.length-1];
     var backObject: GameObject = slabs[0];
     if (!backObject.renderer.isVisible) {
             backObject.transform.position.z = frontObject.transform.position.z - frontObject.renderer.bounds.size.z;
             slabs.Add(slabs.Shift());
     }

}