Never having thought about this I find it interesting. Am I interpreting the numbers correctly? You're saying two people can manage on 51 litres of water for 24 hours which includes showers, toilet, presumably brews and some cooking.
Seems impressively low to me. I would have thought one person would use more than 50 litres for a shower.
Morning all
We could manage on about 30 ltrs a day at a push.

When showering in the van, you dont leave it running constantly.
Switch shower on to wet yourself - switch off - soap up - switch back on to rinse off. Probably takes about 5 + ltrs if done quickly.
The toilet has a seperate flush tank that lasts about 3 days and is filled seperately.
We've never had a real problem using the 40 ltr water roll, but the extra 11 ltr capacity of the new one will stop us sailing a bit close to the wind on occasion. As you mention, the capacity includes normal washing, cooking, brews etc.