I think Wizofoz nailed it
: improved control with degraded or possibly unknown control surface action. For most approaches/landings on a RWY that can reasonably accept your B777, the +20kt speed won't hurt you and it may help, perhaps when you most need it. (Brakes and tires cost a lot less than airframes etc.) Why the difference? Best guess (and only that) is that this is one of the few QRH issues where control may be seriously limited. Again, the +20K gives you more effective authority, perhaps when you most need it. I'm not a 777 driver, but when control becomes an issue, *slightly* higher speed usually helps, so land a little hot, stand on the brakes (4/MAX) and buy new tires later. (If pax aboard, you might also warn them. Not many have been through a MAX braking effort and it can be a shock if not prepared.)