Another explanation might be that they were airbusses.
The Airbus has a function known as Groundspeed mini which adjusts the final approach speed. In essence it tries to keep the final approach groundspeed constant. Consequently if the headwind is 10kts stronger at say a 1000' than it is at the threshold, the aircraft's target speed at a 1000' will be 10kts more than it's planned speed at the threshold.
This means that in lumpy(official meteorological term) conditions the airspeed tends to dance about. In particular the speed 'bug' i.e. the target airspeed can move quite rapidly, so if the mode S readout is based on target as opposed to actual airspeed then I should imagine the effect to be quite apparent.