Having observed numerous rejected take off's in the 737 simulator it has been my experience that on almost every occasion the airspeed kept increasing by around seven to ten knots as the thrust levers were in the process of closing. This was regardless of speed of throttle closure. These were ordinary garden-variety pilots - not test pilots.
OK well maybe it's a cultural difference. Having observed a large number of RTOs in the 146 sim, I have never seen a pilot take more than a fraction of a second to close the thrust levers, but then we always taught that they should be retarded firmly and rapidly. I don't know why you wouldn't just slam them shut, but maybe you have a different slant on that with the 737.
How an engine keeps on producing enough power to accelerate the aircraft when it's only receiving enough fuel for ground idle escapes me though...