I found they have another graph with the same units (written "Wh/mile", so that's unambigious). But it has a different shape, with a minimum at 20-25 mph...
That graph shows a maximum efficiency at around 22 MPH (for the 85 kWh battery), which makes perfect sense -- any faster and air resistance becomes a factor. Any slower and energy uses apart from propulsion begin to eat away at efficiency -- the computer, lights, air conditioning, heat, and so forth.