This happens in my car too, the reason behind this is, when u change the mode to recirculating an window\door of the blower is opened to get air from outside but when the car is at low spped, the blower air is thrown out of this door, so the air circulation is divided and hence felt as low speed (less speed of the air from the blower).
For checking this just turn on the blower to 4\high speed, in recirculating mode, come out of the car and you can hear the sound of air blowing near the wents below the wipers.
Find the picture for reference.