An isogonal is not a great circle. Nothing follows an isogonal from one place to another. It does not represent the shortest distance anywhere.
Isogonals merely represent the points on the earth's surface where the magnetic variation is exactly XX deg E. Move a smidgin to the side of that line, and the variation is now XX-bee'sdick.
What governs that is rather complex, due to the earth's core content mixing.
There is also more than one magentic pole at each end of the earth, but that's another story
