I would say that the internal insert has greater relief. On the outside of a part the surface curves away from the insert. In a hole the surface curves towards the tool so it could rub if you don't have enough relief.  I would guess that an internal insert might work okay for occasional outside threading. Certainly worth a try I would think. If I only had a couple of threads to do that is what I would do.

They are also different "hands"  (L/H and R/H)  , especially on a lathe.
Graham

Yes, you  are right they very well could be.  I thinking in milling terms.