That will be because you are now tapping into both energy sources!
You have a set power to give you a set airspeed.
You then reduce that power.
If you attempted to hold altitude on the reduced power the airspeed would reduce but you let the aircraft descend hence making up for that lost power by tapping into the inherent energy in the airframe.
That is a simplified view as we all know that AOA and drag come into it but the principal is that you have two energy sources available one from the powerplant the other from the airframe.
Take your scenario you have reduced power and allowed the aircraft to descend which maintains that airspeed.
All going well till you hit a rising or sinking pocket of air what happens then?