Below the flat-rate OAT, engine thrust ideally remains constant. But the airplane performance improves with decreasing temp, because the wing is operating in cooler air. Put in other terms, the TAS (inertial speed) associated with V1, Vr etc. is lower than at the flat-rate point. Thus you reach a given V-speed sooner after brake release.
Thus for a given runway, your performance-limited TOGW can be higher, even with the same thrust.
BTW, what is your logic in using T-flex BELOW flat-rate point?