Hi Alex,
I noticed the difference in lighting setup too, while playing around with the scene. I need to talk with the developers to see what is happening because the directionality you point out (bright on top, darker on bottom) doesn't make sense to me, given the all white IBL . I think there may be a bug in the rendering (even w/o the line), that has been fixed in the 2014 version (15.2.2).
I see that you have the 14.2 version so you should at least get the 14.2.1 update as it has significant bug fixes in it, one of which deals with merging 3D layers.
regards,
steve