2x4s are not actually 2 inches by 4 inches. When the board is first rough sawn from the log, it is a true 2x4, but the drying process and planing of the board reduce it to the finished 1.5x3.5 size. Here are the common sizes of lumber, and their actual sizes. Crate Pro default lumber materials include the Actual Thickness and Actual Width for accurate design calculation as well as the Nominal Thickness and Nominal Width for ISPM usage calculation.
Users should adjust these values if they purchase 'true' sized lumber.