i think arrow stick to the outside when there has no enough space inside.
i mean this happen when two arrowheads cannot fit inside between two projection lines.
Don't you want to use another type of terminator like obliques?
It is also possible to use shelves for the very short distance dimensions like here http://www.youtube.com/watch?v=RGq6mpZa8wo
you can also try to play with DTAN attrbiute like
DTAN HOR or DTAN VERT or DTAN PARA and see what will happen
I think DTAN VERT will return arrows as you wish them to see
