B777-200 FLT CREW WAS ISSUED AN OCEANIC CLRNC DIFFERENT FROM THE CLRNC FILED WITH ATC. THE FLT CREW ASSUMED THAT THE CLRNC THROUGH CANADIAN AIRSPACE WOULD REMAIN AS FILED; BUT DISCOVERED FROM CANADIAN ATC THAT THIS ROUTING HAD ALSO SIGNIFICANTLY CHANGED.

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JUST AFTER PASSING FROBAY WBOUND; THE CTLR ASKED US WHAT WE SHOWED FOR OUR CURRENT AND NEXT WAYPOINTS. WE TOLD HIM; AND WERE INFORMED THAT THOSE WERE DIFFERENT THAN WHAT HE HAD ON HIS STRIP. HE SENT US TO ANOTHER WAYPOINT A FULL DEG S OF WHERE WE WERE HEADED; THEN GAVE US A SECOND AND THIRD WAYPOINT; AT WHICH OUR 'NEW' RTE AND OUR PREVIOUS RTE SHOULD HAVE MERGED. WE ENTERED THE NEW POINTS AND LINKED UP OUR RTE AS ADVERTISED; AND THOUGHT ALL WAS WELL. A FEW MINS LATER; HOWEVER; THE CTLR SUGGESTED THAT WE READ OFF OUR ENTIRE RTE TO ZZZ SO HE COULD VERIFY THAT IT ALL MATCHED WITH HIS PLAN. IT DIDN'T. ALMOST EVERY POINT ON OUR FLT PLAN FROM FROBAY TO ZZZ WAS DIFFERENT. AT THAT POINT WE TOOK A COMPLETE NEW ROUTING/CLRNC FROM THE CTLR AND CONSIDERED THE SITUATION SORTED OUT. THE MORE WE THOUGHT ABOUT THE DISPARITY; THE MORE CONCERNED WE BECAME ABOUT WHAT WAS GOING ON; AND WE THOUGHT BACK TO EARLIER IN OUR FLT. HAD WE BEEN ON OUR GUARD EARLIER; WE'D HAVE PERKED UP WHEN WE GOT OUR OCEANIC CLRNC. THE OCEANIC CLRNC WAS 'AS FILED' UNTIL THE LAST 2 POINTS. OUR FLT HAD BEEN PLANNED FROM N65W050 TO N65W060 THEN KENKI TO COAST IN. OUR OCEANIC CLRNC AMENDED THE PLAN TO GO FROM N65W050 TO N64W060; THEN COAST IN AT MUSVA. BECAUSE OF THE INCREASING FREQUENCY OF SMALL RTE ADJUSTMENTS LIKE THIS; IT DID NOT OCCUR TO ANY OF THE 3 OF US THAT THIS COAST-IN CHANGE REQUIRED ANY ADDITIONAL ACTION ON OUR PART. WE ACCEPTED THE CLRNC; PROGRAMMED THE FMC; AND CONSIDERED IT HANDLED. A CLUE (THAT WE DIDN'T PICK UPON): OUR ORIGINAL PLAN HAD TAKEN US FROM KENKI TO YFY. THE OCEANIC CHANGED THE POINT AFTER COAST-IN TO YFB. THEY'RE ALMOST CO-LOCATED; SO WE THOUGHT NOTHING OF IT AND HOOKED YFB TO OUR ORIGINAL ROUTING. IT WASN'T UNTIL WE GOT THE QUESTION FROM THE CTLR THAT OUR ISSUE BECAME APPARENT. UPON REFLECTION; WE'VE REALIZED THE NEED TO VERIFY OUR ENTIRE DOMESTIC CANADIAN ROUTING UPON FIRST CONTACT WITH THE CANADIAN CTLRS; WHICH WE DIDN'T DO. THINGS HAD GONE TOO SMOOTHLY AND HAD BEEN TOO EASY TO MANAGE FOR US TO REALIZE THAT WE HAD A PROB UNTIL THE CTLR APPARENTLY SAW US BEGIN TO DIVERGE FROM HIS PLANNED COURSE ON RADAR. RAMIFICATIONS: THANK GOODNESS WE COASTED IN UNDER RADAR CONTACT. HAD WE COME IN WHERE WE WERE ORIGINALLY FILED; OUR INCORRECT FLT PLAN WOULD NOT HAVE BEEN DISCOVERED UNTIL OUR FIRST POS RPT INSIDE YFB AT WHICH POINT WE WOULD HAVE BEEN MORE THAN 60 MI N OF COURSE (FILED AFTER YFY: N63W080-N62W090 -- CLRED AFTER YFB: N6030W090). NEW CLRNC CONTINUED WITH N58W100; YVC; J505 MEETO; COMPR; STIGS; J540 MLP. FLT PLANNED RTE WAS VERY; VERY DIFFERENT. THE ONLY WAY WE COULD HAVE CAUGHT THIS FLT PLAN ERROR WOULD HAVE BEEN TO VERIFY OUR ENTIRE RTE WITH ATC UPON COASTING-IN. NONE OF THE 3 OF US REALIZED THE NEED TO DO THIS. WE DO NOT ROUTINELY VERIFY OUR DOMESTIC ROUTINGS WITH CANADIAN ATC. AND WE HAD A FLT PLAN THAT SEEMED TO WORK OK. OUR NEW COAST-IN AND SUBSEQUENT FIXES LINKED UP NICELY WITH OUR FPR. AND; SIGNIFICANTLY; PLOTTING WOULD NOT HAVE REVEALED THIS MIX-UP. A PLOT WOULD HAVE SIMPLY SHOWN US ON-COURSE AS FLT PLANNED AND PROGRAMMED IN THE FMC. I'M STILL NOT SURE WHAT HAPPENED HERE. IT WAS AS IF THERE WERE 2 FLT PLANS IN OP: THE ONE WE HAD ON BOARD; AND THE ONE ATC WAS WORKING FROM. THERE WAS A HUGE POTENTIAL FOR A GROSS NAV ERROR HERE; OR MUCH WORSE.