Why does one of the Boeing 777's spoilers deploy differently from the rest?
Roll control is provided by 2 flaperons, 2 ailerons, and 14 spoilers.  

Spoiler panels 4 and 11 are controlled mechanically rather than electrically. They are driven directly from control wheel deflections via a control cable. In case of compete electrical loss, limited roll control is available until the electrical system is restored.  This probably explains the difference in rigging. 

Spoiler panels 4 and 11 are also used as speedbrakes, both in the air and on the ground. The speedbrake function for this spoiler pair only has two positions: stowed and fully extended. The speedbrake commands for spoilers 4 and 11 are electrical in nature, with an ACE (actuator control electronics) giving an extend or retract command to a solenoid operated valve in each of the actuators.

Once that spoiler pair has been deployed by a speedbrake command, there is no control wheel speedbrake command mixing, as there is on all the other fly-by-wire spoiler surfaces.

Boeing B-777: Fly-ByWire
Flight Controls