ACR WDB DEVIATED FROM CLRNC ROUTE ON EXTENDED OVERWATER OPERATION.

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ON AN INT'L FLT FROM TOKYO TO SEATTLE WE WERE GIVEN A CLRNC ON THE GND THAT DIFFERED FROM OUR FILED AND COMPUTER FLT PLAN ROUTE. BECAUSE OF THE ADDITION OF THE POINT APPROX 38 MI OFF OF OUR FILED ROUTE AND OUR FAILURE TO NOTICE THE CHANGE WE ENDED UP APPROX 25 MI OFF OF OUR CLRED ROUTE. WHAT CAUSED ME TO NOTICE THE PROB WAS AN ACR Y FLT BEHIND US RPTED THE POINT WE HAD JUST CHKED EXCEPT HE ESTIMATED A POINT DIFFERENT THAN THE ONE WE HAD JUST ESTIMATED. WE CORRECTED BACK TO COURSE AT THAT POINT W/O NOTIFYING ATC BECAUSE WE DID NOT FEEL OUR DOING SO WOULD CAUSE A HAZARD. I FEEL THE PROB AROSE PARTLY BECAUSE ATC DID NOT IDENT OUR ROUTING AS A REROUTE AS THEY OFTEN DO AND PARTLY BECAUSE WE (THE CREW) DID NOT COMPARE THE CLRED AND COPIED RTE WITH THE FILED/COMPUTER FLT PLAN RTE. ADDITIONALLY; WHEN I ESTIMATED THE NEXT POINT; ATC DID NOT CATCH THE DISCREPANCY. IN THE FUTURE I PLAN TO COPY THE CLRNC IMMEDIATELY NEXT TO THE FILED ROUTING FOR EASY COMPARISON. SUPPLEMENTAL INFO FROM ACN 94002: CHANGE IN FLT PLAN BY ATC CTLR W/O A COMMENT BY HIM THAT THE FLT PLAN WAS CHANGED. GND CALLED WITH A PROB AT SAME TIME CLRNC WAS COMING ACROSS. F/O COPIED THE CLRNC.