A B777 CREW NOTES AN UNCOMMANDED CHANGE OF THE THRUST SETTING TO REDUCED CLB PWR AFTER ENGINES WERE STARTED AND ELECTRICAL PWR SHIFTED TO ENG GENERATORS.

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AS WE PREPARED THE PERFORMANCE COMPUTATIONS FOR THE FLT; WE WERE A RELATIVELY LIGHT B777B FOR OUR SHORT DOMESTIC FLT. THE FO SET THE ZERO WIND ASSUMED TEMP OF 54 DEGS IN THE THRUST LIMIT PAGE. THE THRUST REFERENCE MODE ANNUNCIATION OF THE TOP EICAS INDICATED CLB CORRECTLY. AFTER ENGINE START THE THRUST REFERENCE MODE CHANGED FROM CLB TO CLB 2 ON ITS OWN. THE ASSUMED TEMP REMAINED AT 54 DEGS C. THE PERFORMANCE WAS CLB THRUST LIMITED; NOTE THE C AFTER THE EPR. THE ZERO WIND LINE OF THE ACARS RWY DATA INDICATED: 0 1.31C 84 415.9 130/54 OAT AT ZZZ WAS 44 DEGS F (6 DEGS C) TOG WAS 405 AND ZFW WAS 369. THIS WAS RELAYED TO THE FLEET AFTER THE EVENT FOR INFORMATION. ALSO THIS WAS THE SAME TYPE EVENT; BY ANOTHER RPTR ON ANOTHER P MODEL B777.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT HIS COMPANY DOES NOT USE REDUCED CLB IN ANY OPERATION FOR THIS ACFT. THEY HAVE RECENTLY CHANGED THE ACCEPTABLE REDUCED TKOF TEMPERATURE SETTINGS TO ALLOW THE USE OF ASSUMED TEMPERATURES UP TO 65 DEGS. INCIDENTS OF THIS TYPE APPEAR RANDOM AND ISOLATED AND HE NOTED THAT IN THIS CASE THE SHIFT OCCURRED AT THE POWER SHIFT FROM THE APU TO ENG GENERATORS. THERE IS NO CHECKLIST ITEM FOLLOWING ENGINE START THAT WOULD LEAD TO RECHECKING THE SETTINGS THAT ARE PREVIOUSLY CHECKED AND BRIEFED.