1x1 wall deco mesh complains of left-right intersection

I created a piece of wall deco that is exactly 1x1 blender units and centered in the workspace (shadow plane is the same size/position.) When I place it in game I can tile seamlessly vertically but cannot put two of them next to each other without bb.moveobjects. What is the exact width of a wall segment in blender units? Or how do I size the mesh to avoid overlap when placing side-by-side without introducing gaps?

(Note: when I place side-by-side with move objects they go right beside each other and I don't see the overlap the game is complaining about without the cheat.)

Cheers!