RWD 10yr Festive Season Performance, page-38

So as I suggested it comes down to recovery or assumed recovery. RWD brine has a K concentration of 5850mg/l.
It is very hard tracking down how AMN derived 69%. They did a single 90 day trial evaporating 460L of brine in a Lab/shed in Perth. They don't appear to state a recovery rate in the June 2016 results. In the scoping study they estimate 69%.

Compare this to RWD who did an actual field test evaporating 10000L over 180 days in the open air at Sylvania, Newman, same latitude as LD. This was followed up by a lab trial in Perth.
RWD never stated a recovery rate in their scoping study though a recent presentation suggested 60-70% and brine pumped would be 60-70GL. I suspect RWD knows that the only recovery rates of any value are those done in vivo, on the lake bed.

As you say, the maths is easy. If RWD assumed a recovery of 69%, they could claim they only required 40GL to produce 370Ktpa.
It appears to come back to presentation.