Just my understanding...the wood resins binding the fibers soften with heat, allowing slippage of the bonds when bending. Woods have different fibers and resins and ratios of those components. If the resin isn't heated enough to allow slippage of the bond, the wood springs back. Since the heat is from the outside, inner areas may not be heated enough and they retain their stress. When the stock sets for a while, those stresses may pull the stock toward the original dimension. I've had good luck in longer duration heating when springback was a problem.
