Airlines may used different assumed passenger weights depending on the type of carry on baggage program they have.

For example on United Express EMB-145 and CRJ-200 aircraft, since the operations specifications dictate that passengers MAY NOT carry on any baggage other than personal items, the average passenger weight is assumed to be lower. Any carry on bags that are gate checked are assumed to weigh 20 lbs each. On United Express CRJ-700 aircraft, all the carriers' operations specifications lists those aircraft as having a carry on baggage program. The assumed passenger weights will be higher to account for the probability that more bags will be stored in the cabin and not accounted for as part of the gate checked baggage count.

So for operators who do have a carry on baggage program, getting bags into the cabin is technically a legal way to reduce the "weight" of the aircraft because those bags are considered in the average passenger weight whether a passenger actually has one or not. When they're gate checked its a double whammy. Occasionally larger aircraft end up with weight restrictions. Icing conditions on arrival, inoperative APU, distant alternate requiring additional fuel, all these factors may cause a restriction in the maximum allowable takeoff weight.