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"The object of diving gas mixtures is to reduce the oxygen fraction"
The object of what mixtures for what purposes?
In sport diving, the object was to neither reduce nor enrich the percent of oxygen, but to use standard air.
But standard air "is" close to an 21% oxygen 78% nitrogen mix, and for some 20 years now sport divers have been using oxygen ENRICHED air mixes under some trade names like Nitrox(r). By enriching the oxygen content (raising the percent) you can easily decrease the nitrogen and associated problems of the bends, while still avoiding the problems of oxygen toxicity (and outright flammability) that start getting very much worse around 40% oxygen.
Thw two "standard" Nitrox mixes give you, roughly, one more group each in the decompression tables, or that much more safety if you use those mixes with the old air tables, because they have less nitrogen content and not enough oxygen to become dangerous.