Hi iolofato, so yes the concave shaping that you're seeing is due to "ruling line twisting" like I was showing above (http://moi3d.com/forum/index.php?webtag=MOI&msg=10257.3)

It's a type of shaping effect you can get on a ruled surface (a surface made up of straight lines connecting between 2 curves) if the ruling lines are not restricted.

If you turn on your surface control points you can see yours is similar:



- MIchael