I strongly suspect they use a new generation of cells (same ones as in the 400 mile roadster?)that enabled them to make the battery both lighter and hold more range than the 60kwh in order to achive that much more range, despite the added dual motor weight. If that is true and you extrapolate the 60 to 70 kwh upgrade to the 85kwh battery, you would end up with 99kwh! And more than proportional range gains, because I think the weight of the 85D and P85D is already at the acceptable limit and they will try to make it lighter again, and thus more efficient, too.
(70/60)*85 = 99,16kwh