To keep the function of whatever that map does from turning on and off rapidly at the limit point, there are two limit points. In between the two points, there is no change.
Rev limiters on most of our cars work the same. Say, fuel is cut right above 7000 rpm, but will not kick in until rpm falls to 6900rpm. If it was just 7000rpm, it would turn the injectors on and off very fast right at 7000rpm, which may be bad on the engine.
http://en.wikipedia.org/wiki/Hysteresis