This is related to the diluter demand system, O2 present in the cabin, and tracheal pressure.
The diluter demand system is designed to compensate for the
short-comings of the continuous flow system. It gives the user oxygen
on-demand (during inhalation) and stops the flow when the demand
ceases (during exhalation). This helps conserve oxygen. Additionally,
the incoming oxygen is diluted with cabin air and provides the proper percentage of oxygen, depending on the altitude. This system
is typically used at altitudes up to 40,000 feet.
Source: Oxygen Equipment: Use in General Aviation Operations - Federal Aviation Administration
When you get at altitudes higher than 35,000 feet you're more or less totally reliant on the oxygen provided by the diluter, that's why the diluter demand is very close to the cruise time. If there's low air pressure in the cabin above 35,000 feet there won't be enough oxygen for normal body function, and you run the risk of becoming hypoxic within half a minute.
Toe of Useful Consciousnees (TUC) per Effective Performance Time (EPT)
Look at the chart below:
Tracheal oxygen partial pressure starts at 149mm. Hg for sea level and can drop to 20 at 40,000 feet. If oxygen is given according to the % supplement O2 required for inspired air, the tracheal pressure stabilizes back to what it is at sea level. Above 30,000 feet, tracheal oxygen partial pressure cannot be maintained at 149mm. Hg. Therefore you need to use the diluter longer to at least maintain it close to 149mm. Hg. Or at level where the mm. Hg. isn't extremely low.
The required mean tracheal oxygen partial pressure can be different at the same altitude depending on the liters of O2 per minute BTPS (Body Temperature and Pressure Saturated) being inspired.
Source: Introduction to Aviation Physiology
Fore example, according to 14CFR 23.1443:
(ii) At cabin pressure altitudes above 18,500 feet up to and including
40,000 feet, a mean tracheal oxygen partial pressure of 83.8 mm. Hg
when breathing 30 liters per minute, BTPS, and with a tidal volume of
1,100 cc. with a constant time interval between respirations.
(2) For each flight crew member, the minimum mass flow may not be less
than the flow required to maintain, during inspiration, a mean
tracheal oxygen partial pressure of 149 mm. Hg when breathing 15
liters per minute, BTPS, and with a maximum tidal volume of 700 cc.
with a constant time interval between respirations.
So, at above 30,00 feet you will use most of the diluter demand oxygen to preserve tracheal pressure until the plane has stabilized. At lower altitudes the diluter demand is lower because you can reach safer altitudes faster.