Yes, the speed decrease is accomplished by increased slip.
Operating speed is determined by the intersection of the load (fan) torque speed curve and the motor torque speed curve.
Fan curve has increasing torque for increasing speed.
NEMA B and D torque speed curves shown at Figure 1 here
http://www.ecmweb.com/mag/electric_understanding_induction_motor/ Decreasing the voltage lowers the motor torque speed curve (decreases the torque at any speed).
For a standard NEMA design B industrial motor this is not an effective speed control method because speed can only decrease over a very narrow range below syncronous before breakdown torque is hit.
But, the fan acts more like a NEMA design D motor. It has a very gradually varying torque speed curve from sync down to zero speed. Adjuting the voltage is an effective means for contorlling speed of this type of motor.
