So, as per my Oxygen levels and explosions question, there is a planet with substantially lower oxygen (16%) and a planet with higher than average oxygen (25%) that are fairly earthlike in temperature, locked in a binary orbit around a sun-like star, far enough away that it is possible to travel from planet to planet with 1960 era tech. That was in the past, and it is now near future/2150 tech, and a rebellion of succession from low-planet. However, their tech level is scattered and thus is their weapons, causing lower oxygen to be relevant.
In my original question, I stated that the plants were able to re-absorb oxygen and use it somehow to make energy. Alex-P pointed out that it is not likely for that to be possible.
In short, I need a low-oxygen planet with high plant life and low animals. What is a feasible or semi-feasible reason why the planet has such an oxygen level?