>> But if my method is false your is also false !
Have you made some close up along the curve?

Apparently yes, for the ends of the curve... See message 277.4 by Jonah.
Probably less noticeable as I use a lot of points. And there's a workaround that Jonah also explains...

It may be important for the final object depending if you're look for engineer precision or "visually satisfying" appearance ;-)