Another option that you have in configuring your GPS unit is the heading or north reference units. The better units allow you to select the heading or direction that the unit is indicating in a variety or ways. Yes, north may not always be north. In the settings under program or configure, you will be asked how you wish the unit to indicate directions. True north is not the same as magnetic north.
Magnetic north varies with each longitude and is different each year.
The difference between true and magnetic north is called variance on nautical charts and declination on land maps or topos. Some units automatically figure the variance factor and display an accurate magnetic reading. Others require you to set the variance as it changes and as you move west to east or east to west. Traditional navigation procedures and courses require that the headings be plotted with true north references. This is so because the variance changes each year, and if you were to return to the same area a few years later your true north courses could be easily converted to the changed variance magnetic courses.