B777-200 First Officer reported diverting and making an overweight landing due to unreliable fuel quantity indications.

Narrative

During cruise flight after about 3 hours airborne we were alerted by the aircraft's crew alerting system that we had a fuel imbalance followed by a fuel disagree message. With fuel in the center tank and differing amounts of fuel in the wings the indication of a fuel imbalance was what was being shown on our gauges. After running the fuel imbalance checklist and the fuel disagree checklist we discussed among all 4 crew members the possibility of having a fuel leak. With 11 hours of flight time ahead of us and an ocean to cross we decided to call Operations and start a discussion with Maintenance; Dispatch; and other tech pilots. After looking at fuel flows; indicated fuel values; calculated fuel values; landing weights; fuel pump positions; alternate airport options; and any other related info we found that we could not account for 9;000 pounds of fuel and that the best course of action was to divert to ZZZ to complete an overweight landing (585;000 lbs MLW is 575;000). The flight proceeded without incident and no emergency was declared. After the flight concluded it is believed that we may have had a fuel quantity sensor failure.It appears that there was a failure of an aircraft component so it's hard to say how it could be prevented.