Symmetrical objects are distorted when UV unwrapped.
-Create for example a UV sphere with 8 segments and 8 rings
-Delete half of it (so that you have something that looks like half of an eye, with the iris in the center)
-Select all faces, then unwrap
The UV map is not symmetrical.
-applying the Scale/Rotation does not help
-when unwrapping, changing any of the F6 options (Angle Based, …) does not make it symmetrical
-trying to work around this by pinning a line in the UV map after straightening it with Scale -> X -> 0 does not help, because this will only straighten a line, but the line’s segments will still not be symmetrical