I think you’re getting confused by the fact that Line Trace draws between two coordinates in world space, while this vector(B-A) is not a world location. A mathematical vector does not have an origin, it is just a direction and a magnitude, which means that when you plug in the result B-A, it will appear as if you’re drawing towards the center of the world. This is because you’re still treating it as a world location. For the trace to behave properly, you will have to do this:
Start: World location of Object
End: World location of Object + Vector(B-A)
Try the trace mentioned above, but where world location is from different objects. It will make more sense then. 
Edit*
To clarify. In programming, a vector is just how to store some values. In math, a coordinate and a vector is NOT the same thing. While the values may look identical, they have different purposes. In programming it’s all a matter of how we use values, so we can cheat. So a vector can be used as a coordinate, and a coordinate can be used as a vector. Mathematicians cry, but we can have fun with it. Keep this in mind.