Why doesnt your back of the envelope calculation (did you do it?) match ARES analysis? Could they perhaps achieve high enough mass in a small volume by using e

Why doesnt your back of the envelope calculation (did you do it?) match ARES analysis? Could they perhaps achieve high enough mass in a small volume by using eg. lead? Could it be that a train is more efficient at converting potential energy than a water turbine? Could it be that they have a longer drop? You gotta show your assumptions if you're arguing through back of the envelope calculation.