Actually, the rpm rises because the rotor thinks the weight of the helicopter went up - the positive G that builds because of the increased thrust is exactly the same as if the aircraft suddenly got heavier. If you pull 1.2 G as you flare, and you do, that is as if you added 20% gross weight, as far as the rotor is concerned. This means the trimmed rotor rpm must be higher, too (rpm goes up as weight goes up, for a given collective pitch position).
Another way to look at the same effect is to realize that the inflow velocity is raised considerably. It rises because we force the rotor to eat more air, which propels the system to higher rpm's.