getRaySource() inaccurate?

I have a sphere in the center of a screen. Using blendenzo’s tutorial i have created a mouse. It works great. However, when i click an object, i have a script print out the location of the center of the object, and the RaySource() of the mouse (mouse position). However, even when clicking as near as possible to the center of the object, i’m getting reports that the x-position is off by 3 units. In fact, clicking anywhere on the sphere reports a difference of about three units. What’s going on?