Ironbutt57,
The word certified is misleading, as the certified MLW will always be lower than the certified MTOW. However, FAR25-1001 (I believe) states that an aircraft must be capable of returning to land within 15 minutes of takeoff. There is no way that a B757 or any aircraft can burn off the fuel difference between MTOW and MLW in 15 minutes, therefore it is taken that the aircraft can land at the MTOW.
If you look at a B757 landing chart, the weights shown will be much higher than the MLW for this reason.
Mutt.