Lighter pack & motors, and the battery and motor are optimised for lower power operation so the efficiency of the system is higher.
It's also possible the 85/90 are cycle-derated to make the 70 seem better, for example the 90 could be capable of more miles but the range computer estimates much less.
Tesla does this the opposite way around for the 70kWh Model S. It's capable of something close to 250 miles but it's derated slightly to 240 miles to make the 85 and 90 look better.