Wood swells as it absorbs moisture and shrinks as it dries. This can change the thickness and width of the wood considerably, but changes in length are minimal. As such, tolerances in wood dimensions are natural and not a defect.
A very natural consequence of this swelling and shrinkage behavior is checking (cracks) and warping (deformation). Checking is most common in wood where the pith is still enclosed (as in fence posts). Checking does not affect the stability or durability of the wood and can close almost completely, depending on the weather. DIN 4074 tolerates checking as long as it does not adversely affect the durability.
If you are building a project with freshly sawn (= undried) larch/Douglas fir, you should expect at least 4% shrinkage after processing. As a result, carefully and well finished structures may appear less attractive after some time. You can prevent this by using kiln-dried larch/Douglas fir. Wood is a natural product that can swell and shrink. As such, all sizes listed in this catalogue may deviate slightly.