That's happening because placing objects in custom edit mode doesn't actually detect the surface under the mouse, instead, you actually place the object on a flat XY plane and if it is below a surface it then moves up or down on the Z-Axis. At low camera angles, it can be rather confusing and you'll probably end up placing objects some distance from the objects instead of where you expected them. I'll pass it along to our development team to see if it can be improved.
All the best,
Paul