The 757 issue, from what I understand, has to do with the load limit of the floor in certain sections of the airplane. When the average passenger weights transition to winter weights on November 1st, you can no longer have all seats filled and still comply with that limitation. Depending on the aircraft configuration, either three or six seats, all middle seats, are blocked and unusable on the affected aircraft during the period that winter weights are used. Each blocked seat is marked as unusable and can not be occupied for any reason.

General weight restricted flights have already been discussed. In those cases, the determination is made based on average passenger weights and have nothing to do with a particularly passenger's actual weight.