What you need to know
Wood is not a man made material. It does not have the stability, consistent quality or predictable properties of say plastic or concrete.
Wood is affected by it’s environment. It shrinks it grows it bends it twists and can even split., all depending on the humidity of it’s surroundings. Furniture that has lasted 100’s of years in a stately home can crack and warp in a modern centrally heated home in as little as a few weeks. This is due to central heating sucking any moisture out of the piece causing shrinkage.
Most products for the home use wood that has been Kiln dried, ie. dried in an oven. this aids production as dryer wood is easier to machine and prevents loss of moisture in modern homes from causing defects.
Wood used for exterior applications however is generally not kiln dried, it would be pointless as wood will naturally try to absorb moisture.
See the video on Pressure treated timber