Proportion deviations are rarely accidental. The decision on how to cut the rough crystal is very complicated and depends on many economical and practical factors. However most diamonds are cut for maximizing weight yield from the rough stone at the expense of beauty.
Antwerp Ideal cutters cut for beauty only By adhering strictly to the mathematically correct angles. Rough diamond crystals occur naturally in many different shapes. Sometimes they are found in their symmetrical crystal shape "the octahedron". This rough crystal can yield two stones and must be sawed or cleaved.
Few rough diamonds are in a perfect natural crystal shape like this octahedron. Most rough is irregular and distorted.
Cutting a larger table, shallower crown angles, or both, produces a thinner crown. This usually saves weight and gives the stone a larger girdle diameter at great expense of fire.
The cutter cutting the Antwerp Ideal will cut two well proportioned stones, one large and one small.
With distorted rough there are also numerous ways to save weight, again by fudging slightly on the proportion and angles of the finished diamond. Depending on the rough diamond's shape, cutters can cut a Shallow or a Deep stone.
Antwerp ideal cut
The blue outlines represent the Antwerp Ideal proportions. The red areas indicate where the average cutter is most likely to retain extra weight. Increasing the girdle thickness is another favorite way to increase the finished weight.