'Overboost' is used to describe the turbo being allowed to boost above the normal pressure for a short period (in the 5-20 seconds range) after the accelerator is fully pressed. After this period, the boost is dropped down to the 'steady state' level which is where power and torque measurements are taken.
Overboost is allowed by the ECU since a turbo engine can safely produce more torque for a short period without blowing up (well, wearing out). It only is possible since the ECU is limiting the boost for engine or transmission longevity (or to make the car nicer to drive).
In general, only the torque is quoted at the overboost level, since it is only when accelerating when it is really effective. And at full power revs, it is likely the boost isn't limited by the ECU and so there isn't any overboost available.
And I've even found a power/torque 'curve' showing the data for the Cooper S and the overboost does disappear once the boost isn't ECU-limited: