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.
The difference is real and documented on the Model S, and is greater than the weight alone can account for.
I asked about this difference (on the S) a while back, and got a lot of numbers supporting the EPA data and a few theories for why, but no one seemed entirely sure.
I think the X architecture is similar enough to the S that the information provided penalty does accurately reflect the relative range - which makes choosing harder, because not only is it "only 37 miles" for $13k, but you actually have to use ~10% more power/pay more money for all the typical days you aren't on a long trip.
The 70kWh battery has 14 modules and the 85kWh battery has 16 modules. Same small batteries in both. The 90kWh changes the chemistry to add 6% range but uses the same 16 modules.
Using the EPA figures for the X:
220 miles * 16/14 * 1.06 = 266 miles of range
The reason the EPA lists 257 miles of range is due to increased weight and also increased power (as EPA testing accounts for faster acceleration). This results in a vehicle which is 3.7% less efficient than the base model 70D because of weight and acceleration.
Don't get caught up on the exact size of the battery; by using the module count you can see how the numbers really work. 70 and 90 are not exact figures.
85kW.h battery has 16 modules in series with each module having 6 cells in series by 74 in parallel (7104 cells). 6% higher amp.hour cells are used to create the 90kW.h. The new cells must deliver 3.3a.h compared to 3.1a.h for the older cells. However, both cell types have a higher nominal amp.hour rating (3.5 a.h and 3.3 a.h respectively). Thus both 85 kW.h and 90 kW.h batteries have 74 cells in parallel. 70 kW.h battery would suffer a 12.5% voltage drop if the 14 modules have a similar architecture to those in the 85.kW.h - 6216 cells (84Sx74P). The modules may only have 70 cells (total 5880 cells) in parallel otherwise the 70 kW.h battery would have 74 kW.h.
3.6V per cell is the midpoint of the cut-off voltages (3.0V and 4.2V). The voltage difference between charged and near empty charge is 30%. The current needs to go up as the battery runs down to compensate for the voltage decline.