Here is another similar example this time with surfaces being generated.

If you start with these 3 curves:



Then loft just these to get a surface:



That surface will form a crease when mirrored:



However, if you create the whole set of section curves:



And do a loft through all of these as one larger loft, you will get this smooth result:



The same effect will happen with other kinds of surfacing tools that build surfaces through your shapes, like Network for example.

- Michael