Why was the Graf Zeppelin's endurance when fully fuelled with both fuels less than the sum of its endurances with each fuel separately?
Graf Zeppelin LZ-127 carried 75,000 cubic meters of hydrogen and 30,000 cubic meters
of Blau gas, which has a similar density to air.  This helped them avoid buoyancy issues when fuel was consumed.

With gasoline, the extra weight had to be compensated with more hydrogen, hence less room for Blau gas.

But I wonder if they could burn some of the hydrogen as fuel instead of venting it when the gasoline was being used up.  This would increase the range considerably.