Two possibilities. 1)Ultra violet rays have caused a color shift in the finish, and 2)dehydration of the wood and the finish will also cause the finish to be lighter. A simple test for this is to wet your finger and touch the light area. If the wet spot is now darker, then dyhydration is the problem and that can be reversed with Wood Elixir. If the wet spot stays light then the pigment in the finish has been bleached by the sun and you must add a tinted finish to match the surrounding finish.