You'll get a miter if you are doing a one-rail sweep and you've got a planar rail that has sharp corners in it.

You don't get miters on a 2-rail sweep which looks like the case here, if you do a 2 rail sweep on rails that have sharp corners, it's basically the same as cutting the rails up into pieces and having the sweep done on those pieces in separate sections.

Mitering involves doing an extension of the surfaces and intersecting those and in a 2 rail sweep it becomes unlikely that the extensions actually even cleanly intersect one another.

- Michael