Remember that the rated Wh/mi for Model X is higher, so the number of miles gained will be lower for any given number of Wh put into the battery.
Also, the Model X seems to be more sensitive to higher voltages. For me, it will back down the charging current to guarantee the charger stays below 17.5 kW. My home runs at ~250V, and while the display shows 72A before plugging in, it'll drop to 69-70 when I plug in, based on the voltage at the time - unlike the Model S that will charge at 79A @ 250V all day long.