I had to look this up, and I found the answer interesting because I used to develop software to accurately report magnetic declination at any point in time, for directional drilling oil wells in Canada. https://www.geomag.nrcan.gc.ca/mag_fld/magdec-en.php has some great information on why. For example, Yellowknife can experience declination changes over one degree every three years.
I haven't used a compass since I was a kid and at that time declination where I live was 21 degrees. Now it's down to 13 degrees. I had no idea it changed that much.
