Don't forget there are other variables that are considered in setting the scheduled times but the facts on the ground at the time may change. For example, stronger tailwinds then normal will allow an aircraft, all else being equal, to reach the destination faster than anticipated. Amount of expected air traffic, time built in for taxiing and waiting for take off, etc. can all change based on specific set of circumstances. Some days, I land at ORD on a domestic flight and we are practically right at the gate. Other days, they use the runway out by the international terminal and taxi can be loooong. These factors are why different flights between the same origin and destination can have variable trip times, even when the same aircraft is used.