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This graph shows how on open-circuit air the percentage of oxygen in the breathing mix remains constant at about 21% oxygen at all depths (NB: at the same time, the partial pressure of oxygen or ppO2 varies, ie. 0.21 bar ppO2 at the surface, 0.42 bar at 10m, 0.63 bar at 20m and so on).
The opposite occurs with the Inspiration. Because the ppO2 is maintained at a constant level (ie. at the Inspiration default Setpoints of 0.7 bar at the surface and for the descent and switched over to 1.3 bar for the rest of the dive*- as shown in the graph above) the percentage of oxygen in the mix varies with depth. This is because oxygen is injected into the mix within the breathing loop in order to maintain the ppO2 at the Setpoint. For example, on a Setpoint of 1.3 bar there would be 100% oxygen at 3m (ie.at 1.3 bar ambient), 81% at 6m, 50% at 16m, 32% at 30m and 21% at 50m.