It's purely a difference on paperwork. Companies get charged to overfly different people's airspace and those charges are based on the MTOW. If you don't need all of your planes to do long routes then just have some certified to a lower weight by the manufacturer, all the planes are physically identical so could actually carry up to approx 115000 kgs which I think is the max Boeing themselves will certify for a 757-200. They can be re-certified by Boeing for a (big) cost!
