Question about Really Useful Box storage box

These are the external dimensions. According to the manufacturer, the inside dimensions are: 39.5 cm x 33.5 cm x 17.0 cm. Two other features of the box add to the reduced volume. Firstly, the interior of the box is slightly wider at the top than at the bottom, for example, the width at the top is almost exactly 34 cm, but at the bottom it is only about 31.5 cm. Secondly, the inner transition between the bottom and the side walls is rounded (which also makes measurement somewhat more difficult).