Marking a seam makes no difference - it still does it. The Sphere projection option doesn’t appear to respect the seam (the screenshot below was taken after unwrapping with a vertical edge loop marked as a seam). The polar triangular faces dont appear to be cut along the same seam as the quads around the midsection - even if you explicitly mark it.
I have no particular application in mind - it just struck me as an odd thing for it to be doing. For example - if I do try and wrap an earth texture onto the sphere - the offset causes a twisting of the image at the poles.
Personally - it would have expected blender to unwrap a sphere a bit like this - splitting the triangular faces (or at the very least aligning the triangular faces with the square ones)
You can kinda achieve the triangular face splitting by selecting each face in turn and grabbing the vertex in the UV editor. But the offset still hampers this meaning you end up with one large triangle that crosses over.