The reason is that the nominal outputs considers steam and not gas which is actially 2x higher.
As an example: you should always start out from the nominal input and for a large rose gold turbine that is 42000 l/s. For large gas turbines (and also high pressure steam) the nominal output will be 42000/20= 2100 eu/tick. The next step is to calculate the flowrate required per tick
(2100+32-1)/32~= 66,6 mb/t which is rounded down (always) to 66 mb/t. (thanks to sprangles for this one)
This means you need 66mb/t of biogas for a nominal output of 66*32=2112 eu/t or an actual output of 2112*1.2=2534 eu/t.
TLDR: Double the amount of distilation towers you are using or switch to a 21000 l/s turbine like titanium, tungsten, red steel, manganeese, molybdenum, stainless steel or neodymium
. If i would choose i would go for either titanium or tungsten as they have pretty nice durability and are easy to come by. If you wanna pick some other material i would advice you to have a look in my google docs document
On making oxygen: Centrifuging air rather than electrolyzing water is both a lot quicker but also consumes about 25 times less power. Cheapest way to get hydrogen is by electrolyzing methane cells, however if you dont have enough to suit your needs with that you will need to electrolyze water. In the case that covers your oxygen needs thats fine but in any other case i would go for centrifuging compressed air.
If you have a very large need for hydrogen i actually think it would be well worth it to make methane farm to produce hydrogen as for instance 1 carrot is equal to 72*0.8= 57,6 mb of hydrogen