A B777-200 FLT CREW DEVIATED FROM THEIR ASSIGNED OFFSET COURSE OVER THE NORTH PACIFIC.

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WE HAD DELETED A NON RPTING FIX JUST PRIOR AND NEEDED TO RE-ESTABLISH OUR ONE NM SLOP. FOR THE SAKE OF SMOOTHNESS; I RE-ENTERED R1 ON THE RTE PAGE APCHING FIX; EXECUTED IT JUST PAST. IN LNAV THE ACFT MADE A MUCH MORE AGGRESSIVE TURN TO THE R THAN NORMAL (25 DEG BANK); AND FLEW THROUGH THE OFFSET COURSE ON A 45 DEG ANGLE; AND CONTINUED TO TURN R; BUT SLOWLY NOW. I WENT TO HDG SELECT TO TURN BACK TO THE DESIRED TRACK. SHORTLY THEREAFTER; THE MAGENTA LINE DISAPPEARED FROM THE MAP (OFF SCALE; R) AND THE FO STATED THAT THE NEXT WAYPOINT WAS GONE. THE FMC WE NOTICED WAS SHOWING THE COURSE FOR THE NEXT LEG. THIS WAS ABOUT 40 MI TO OUR R. WE REBUILT THE ORIGINAL COURSE AND RE-INTERCEPTED: BACK TO NORMAL. WE RPTED THIS WANDERING TO ATC VIA CPDLC; ESTIMATING THAT WE GOT 8-10 MI R OF COURSE WHILE MANEUVERING BACK ON COURSE. THEY ROGERED. SINCE WE WERE WATCHING; I FEEL THE DEV WAS MINIMIZED; ONCE WE REALIZED OUR COURSE LINE WAS GONE. ADDITIONALLY; NOTHING RPTED HERE ABSOLUTELY ELIMINATES HUMAN ERROR; BUT I CANNOT THINK OF WHAT IT COULD BE. I BELIEVE THERE WAS NO TFC CONFLICT AS WELL. MY QUESTION IS: WHY WOULD THE ACFT JUST BLOW RIGHT THROUGH THE R1 OFFSET? THERE MUST BE A LOGICAL EXPLANATION; BUT I CANNOT THINK OF ONE.