Then think about how many grams of "air" there is in a bottle. One mole of a gas will expand to 22.4 liters at room temp. If there is 1 liter of air in the gallon bottle, then there is 1/22.4 mole of "air" in there, and air being mostly nitrogen and 21% oxygen, we can say there is about 1 gram of air there, so about 1/5th of that is oxygen. So about 0.2 gram of oxygen. It doesn't seem like much of a stretch to me to have the developer react with 0.2 gram of oxygen. THat would be all 21% of the oxygen in the bottle. Not so much when you think of how many grams there are.