Can anybody tell me why I sometimes get the option of making splines symetric and sometimes not.

I draw to splines an a center. When I cntrl select all three line:

Some times there is the option of making it symmetric relations.

Some times there is no option of making it symmetric or any other relations.

Of course if I mirror one of the splines over the center line they become symmetric, but doing so some times

gives me trouble when I am trying to do a filled surface later.

But why is there some times no option of making it symmetric or any other relations between them?

Cheers!