Lately I have noticed that Skyvector's Flight Planning tool "Nav Log" has been showing very wrong magnetic variation corrections for particular legs of my flight plans. The corrections appear to be egregiously wrong and this has been going for at least a few months (since I've noticed). That combination makes me doubt myself, since I'd imagine it would be fixed by now.
I created an example route to show what I mean. I made the route KFRR to KLVL which goes along the 10W variation line:
I included some waypoints on the way to see when Skyvector would give bogus magnetic variations. And sure enough this is the flight plan it reports:
I highlighted in red the legs where it gives a +6 variation correction instead of what should be the correct +10. Is this just simply a bug or am I missing something here?