777-300s with EFBs for performance calculations have an "Alternate CG" selection. If the actual CG is 26%MAC or more, selecting it will increase Performance limited MTOW, as well as increasing the allowed de-rate.
As previously mentioned, it's because it allows for the fact that with a more rearward CG, the downforce produceded by the stab is less, therefore increasing the available lift at a particular speed.