It's actually based on usage time at those speeds. Previously they only split it into two speeds but have made it four speeds starting about a year ago I believe? They do this because the state doesn't allow non-utility providers to sell electricity. So they charge for the time you are plugged in, and adjust by the rate of power draw.
Seems like a bunch of broken systems. Probably goes back to rental landlords price gouging their tenants and the government smacking their greedy little hands.
Tesla and other EV charging providers have come up with this workaround and it seems to work.
*Shrug*