Hi Pilou, which particular side happens to get picked as the seam location might be different depending on the pick order. I had previously wrote on another thread that there wasn't any difference depending on pick order, but I guess this can be an exception to that for some cases at least (probably for when the first curve's natural seam also coincides with the "twist minimized" seam location as well).

But why are you concerned about it? It doesn't really make any difference where the surface closure seam is at.

If you like you can move it by dragging the points that you see inside the viewport, but there isn't really any reason to do that.

If it helps to clarify what you are seeing, you can use the "MarkCurveStart" plugin (http://moi3d.com/forum/index.php?webtag=MOI&msg=5288.21) to place a point at the closure point for the edges you are blending between. You will likely see that one of them has its natural start position at that location where you get the blend surface closing seam.

- Michael