Was the wood the same color to begn with? Does one side get more sun than the other? You can use wood bleach (after sanding
back to bare wood, grrr). Oxalic acid, I think. I've also had good luck using TSP (the real stuff.....tri-sodium-phosphate.....available at paint
I use Cetol light - am very familiar with it. I got over-zealous for awhile......I think I applied too many coats......it gets darker with each additional coat. So you may want to put more coats on the lighter side.
Or.....you could do what I do, and throw up your hands and delight in the strange and unpredictable color variations in natural wood.