Hi nos, this has to do with the starting normal being chosen differently from the vertical line and the arc - basically curve flow works similar to sweep and there is an x/y/z axis frame that slides along each curve. In this case the starting x axis direction is being chosen differently for each curve, so that's what produces a different result.

That is something that I should be able to tune up, I've added it to my list of things to tune up.

In the meantime if you rotate the whole thing down to the world x/y plane it should then work ok for you, see attached file.

- Michael