The nature of timber, being a natural product, means no pieces are exactly identical. It is not a man-made product made from a consistent formulae like steel or plastic.
Different pieces of the same size wood will move differently, causing cracks and twists in some and not in others. Timber placed outside continues to contract during the initial period and then expand and contract throughout its use. This natural movement is even more noticeable during extremes of wet and dry periods.