verify? But generally when computers control things -- 2 signal lines would allow 4 conditions to be set if using just simple binary logic states (all referenced to ground). Both lines off - state 1, one line on and the other off - state 2, second line on and the first off - state 3 and both lines on - state 4.
So 3 speeds: 1st speed with line 1 on, 2nd with just line 2 on and then with both of them on speed 3. FYI: A three-way light bulb in a lamp does the same thing with just 2 filaments.