B777 LANDS AT ZZZZ WITH FUEL LOW EICAS MESSAGE DUE TO WX AND ATC REROUTES.

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I WAS ON BREAK WHEN CAPT (PF) DEVIATED AROUND WX AND AN AREA OF TURB WHEN IN CANADIAN AIRSPACE WITH MONCTON CTR UNDER RADAR CTL. PF ALSO REQUESTED AND RECEIVED CLRNC TO DSND TO FL250 FOR SOME TIME AND THEN BACK TO FL360. CAPT BELIEVED WE WOULD GET A NEW FLT ROUTING INTO GANDER OCEANIC AIRSPACE BECAUSE DEVS LEFT US S AND E OF PLANNED COURSE. GANDER CTR WOULD NOT REROUTE US AND REQUIRED US TO GO BACK TO THE ORIGINAL ROUTING. BY THE TIME I GOT BACK FROM BREAK WE WERE APPROX 8000 LBS OF FUEL BELOW FLT PLANNED FUEL AT APPROX 40W. BY THE TIME WE REACHED SHANNON CTR WE WERE ABLE TO CLB TO FL410. ON ARR ROUTING TO ZZZZ WE GOT A FUEL LOW MESSAGE AND RAN THE CHKLIST. THE CAPT DETERMINED ZZZZ WAS THE NEAREST SUITABLE ARPT AND WE CONTINUED TO LAND THERE WITHOUT EMER PRIORITY HANDLING (ALTHOUGH ZZZZ KNEW OF OUR FUEL STATUS AND SENT US TO THE ARPT WITH NO DELAY). THE FUEL LOW MESSAGE CAME UP BECAUSE THE L TANK WAS APPROX 2000 LBS LIGHTER THAN THE R TANK AND ITS QUANTITY TRIGGERED THE LIGHT EVEN THOUGH WE HAD NEARLY 2000 LBS MORE THAN RESERVE FUEL WHEN WE LANDED. SUPPLEMENTAL INFO FROM ACN 717054: APPROX 500 NM S OF NAT ENTRY POINT CARPE; CAPT ELECTED TO MAKE EXTENSIVE ALT/AREA DEVS FOR SIGMET; ISSUED BY MONCTON CTR. DURING DEV; I ADVISED CAPT THAT WE WERE BELOW RSV+ALT FUEL FOR DEST (9.0). LATER; DURING DEV; I ADVISED HIM OF EMER FUEL STATUS FOR DEST. CAPT CALLED DISPATCH AND OBTAINED A NEW RELEASE ALLOWING FOD OF 8.0. THEN; WE CLBED UP FROM FL250; AND PROCEEDED N 320 NM TO ENTRY POINT CARPE. WE ENTERED THE NAT ALMOST 9000 LBS BELOW OUR ORIGINAL FLT PLAN. APPROX 5 HRS LATER; 120 NM FROM ZZZZ; EICAS AUTO ANNUNCIATED NON-NORMAL CHKLIST 'FUEL LOW.' CAPT ELECTED TO CONTINUE TO DEST. TWICE; I ADVISED HIM TO DECLARE EMER. HE ELECTED NOT TO. LNDG WAS MADE WITH GPWS OVRD; AND FLAPS 20 DEGS (LOW FUEL CHKLIST) FUEL AT LNDG WAS 2.6 (L WING TANK) AND 4.7 (R WING TANK) AND TOTALIZER 7.3.