As a board dries, it can cup.   Green boards cup more than dried boards, but dried boards still cup a little.   Boards cup toward the bark or outside of the log.  

Many many years ago (I was about 17), I was helping my grandpaw (a real carpenter) do finish work on the house at our family's farm.   We were putting up a mantle obove the fireplace.   This mantle was basically a pine 1x12 with some fancy supporting woodwork.   Grandpaw showed me the end grain of the 1x12 and said the board would cup away from the heart of the log as it aged, exacerbated by the heat from the fire.   He wanted it to cup upward so things placed on the mantel would not roll off.   So he turned the barkside (outside of the log) upward.