Heartwood is difficult to impregnate with wood-preserving chemicals. Sapwood is easy to treat. The outer layer as you might guess grows later in the season. Repeated cycles of wetting and drying sometimes cause the layers of earlywood to separate from the layers of latewood. Here we are dealing with appearance and probability.
Knots originate at the center of the tree.
Knots sometimes show up on the pith side of the board, but not on the bark-side surface. In fact, you are less likely to have exposed knots on the surface of a deck if all boards are laid bark-side up. Sometimes you see bark still clinging to an edge. This defect is called wane. It's no secret that the dimensions of wooden objects are influenced by relative humidity.