Why does the oxygen duration chart show increasing times above 30,000 FT MSL?

I asked this question on the biology site. Some suggested I ask this question here.

https://biology.stackexchange.com/questions/95588/why-does-this-oxygen-duration-chart-have-increased-times-above-30-000-ft-msl

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This oxygen duration chart from an airplane shows the amount of time a pilot can be on oxygen at a specified altitude (in 1000s of feet). For example, the chart shows two pilots able to cruise at a cabin altitude of 35,000 FT MSL for 182 minutes using a diluter demand system or 192 minutes using 100% oxygen.

I have one question.

1. Why does the oxygen duration chart show increasing times above 30,000 FT MSL?