When i add Modifier "subdivision surface" it will happen.Don't know why..
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There is a few things you can do.
When you subdivide cube you actually gets a sphere with different topology. Or you can delete the top polygons and grid-fill them, but that needs some tweaking as the grid-fill will be flat, so you can select the center vertex and with the proportional editing and move him little up so the sphere is round.
Also you can use ISO sphere in the mesh menu but that is made out of tris polygons.
This problem on the surface (when using a Subdivision Surface modifier) is caused by the triangular faces on the top.
Repeat for the other pole vertex.