Mirrored object stays in the same place I try to mirror an object [ correction: Lamp ] along global axis, yet it stays same place no metter what axis I choose.

Anyone has any idea why this might happen?

Edit: Object is slightly offset from global Y axis, so what I expect is to see it on the other side of axis when I mirror it, but it stays in same place.

I wanted to go from this:


To this, in one step:


Thanks
I did what I wanted to achieve by setting Pivot Point to 3D cursor and then mirroring. Still doesn't make sense to me, when I execute a mirror around a global axis I expect to see my object on the other side of that global axis, without playing with pivot point.

EDIT:

Please check this screen shot, 3D manipulator widget has an orientation selector (in this case selected to Global) and Mirror Transform also has an Orientation selector (Local). What are their effect for mirror transform. 3D manipulator has an effect at all?