syncing iPhone always copying music files

When I sync my iPhone SE with my iMac, it always copies almost all of my music files (but not all) over to the iPhone. Even if I haven't changed anything in the Music app (iTunes) on my iMac, it still copies about 500 songs. Since it makes the syncing process take that much longer, I would like to find a solution to this. Does anyone have any ideas how this can be fixed?

Edit: Even though below my post says I'm using OS X 10.11, my iMac is actually up to date (10.15.6).