It's not your calcs, that does indeed add up to 90 litres.
I think it's the way you've measured it.
If you take a side view it's not a 390 x 320 rectangle because the back of the tank has a big slope on it under the spare wheel.
I don't know what the dimensions will be but you have to take a big chunk of volume off for the slope.
My slope is accurately measured at 100x40mm (& quoted above)so that only knocks a couple of lts off max
