An aircrafts flight
envelope must consider the strength requirements of the structure in still and
turbulent conditions.
Consider an aircraft manoeuvring at it normal limit of 2.5g in still air. If
the aircraft were to experience a sudden up-gust this would increase the angle
of attack, increase lift and load factor.
The aircraft must be able to withstand this increase hence the factor the gust limit can be higher than the normal limit.