It may be worth noting that this method (using the text alignment button) is how you centre an inline item (an item that can stand on the same line as text) such as an image. If you needed to centre a block item (a container box), you could do this by using the Align popup in the Margins and Alignment section of the Box Inspector.
E.g.
- Insert a box
- Give it a width that is smaller than the width of its parent
- Choose Center from the Align popup in the Margins and Alignment section