Why does magnetic declination change with area?
Imagine I put two dots on a map and ask you what the angle between them is.
After some thought you should accuse me of asking a poorly formed question, because the angle between them depends on the location from which you measure it and can be anywhere between 0 and 180 degrees.
So, if we treat the rotational pole and the magnetic pole as two dots on the globe does it make any more sense to ask what is the angle between your shortest routes to them without first specifying where you start?
The magnetic declination is the horizontal deviation of the direction of a magnetic needle's north pole from the geographic direction of the north pole. There is a wide variation of the magnetic declination over the earth's surface. See Wikipedia https://en.wikipedia.org/wiki/Magnetic_declination .