It seems after reading the report, maintenance did an extensive check of all pertinent systems and found no faults, then did an extensive flight check trying to duplicate the problem and found everything normal. Trying to duplicate the lax landing profile they found that by keeping your hand on the speedbrake handle while deployed it was possible to restrict the left throttle from advancing with just 1.5 lbs resistance to that thrust lever. This procedure was standard after the Cali crash. If the auto throttle was on and the speed knob was set above or at the speed the ac slowed to the throttles would advance even though the pilot didn't intend for them to since the speed brake was out. They decided the FO's left arm might have restricted the left thrust lever inadvertently causing the delay in spool up. Sometimes with all that automation you have conflicting results you don't expect but sort it out when it happens. I have seen similar things happen dozens of times but it is easy to fix once you see the conflict. The 777 automatically adds rudder with assymetical thrust so it would be difficult to notice with no yaw.