Learning about rebreathers is what really explains it. You don't actually consume any more oxygen at depth (besides work load and body heat generation, but those are secondary and not related to depth. The oxygen is consumed, pretty much burned with fats and sugars as the hydrocarbons are broke down and turned into CO2 and water. Sugars are still a form of hydrocarbon even if they have oxygen built into them. That process doesn't change.
In very crude numbers, you inspire 20% oxygen, exhale 15% oxygen and 5% CO2.
If you were pull a gas sample while diving air, the inhale is still about 20% but the exhale side would shift to a much higher O2 and lower CO2. You are simply blowing air through your lungs without burning the O2 off in your body.
This is also one of the theories of oxygen toxicity, getting too much O2 partial pressure into your body and the oxygen wants to react with something. Starts going after nerves and stuff. People talk about long duration high O2 partial pressure (hard core commercial or technical divers) as "burning there lungs". Advanced Nitrox or rebreather classes for this stuff on the recreational side.
Getting deeper into the technical side of diving the mixes are hypoxic, 10% oxygen is fairly common. Not enough to breath safely at the surface, but going real deep it might still be too much.
Thankfully this has been researched fairly well. In normal recreational types of diving there isn't an issue. Lots of other problems before O2 is an issue. Add in some Nitrox, you are on the edge of needing to think about it. That is why there are classes on Nitrox. Then hit the technical side of diving and it is a thing to worry about. But that is why there is good training with the technical classes.
