Expanding shape of a polygon results in change of centerpoint

While it is a shape the centre is at the logical centre from how the polygon is constructed. When the shape is expanded it is converted to paths and the centrepoint is now the mean centrepoint of the bounding box. I do understand that if you are intending to create certain shapes using a polygon that can be frustrating, grouping the shape with a circle where all corners of the polygon touch the circumference would make the centre of the circle the logical centre. keeping that circle as a construction would help to find the centre. 

With a normal path it's the center of the bounding box. This is not necessarily the geometrial center of the polygon.