Sealing wood, particularly the end grain, slows down movement, why you want to seal the ends of planks before drying. Slower movement makes for more residency and less cracking. Presumably it takes longer for the air to reach the inner material, so the wood changes less abruptly. There is usually a differential between the amount of moisture in wood in the outer layers, and what the water content is deeper inside. It's reasonable stable when it reaches an equilibrium throughout, but if you take a "dry" thick board and slice it down the middle, it will often cup because of having the higher moisture content inner surface suddenly exposed to the air. The freshly cut side is wetter than the other side, or in a very wet time of year, I guess it could be drier inside. Even a multi hundred year old piece of wood could still have 6 or 8 percent of water inside of it, picked up from the air. if you were to completely seal the wood, like with some kind of thick epoxy type finish or something, theoretically the wood not being exposed to the air would remain stable at certain dimensions, but I don't think it ever becomes completely stable and not moving when in wider dimensions. Possibly it still responds due to changes in temperature, or few finishes are completely impervious to air.
Wood might be resistant to moving if enclosed in glass and kept at a constant temperature.
