So thats the thing ablut UV unwrapping the UV Sphere, because the lengths of the edges are allowed to be distorted, what should be closer to trapezoids making a bunch of slightly rounded slices can be distorted slightly to squares/rectangles without real significant distortion when applied to the object with the exception of the triangle fan at the very top and bottom.

If we were unwrapping this to completely minimize distortion at top priority, a UV sphere would look closer to the Polyconic Map Projection.