Everytime I hook up my ipod, it syncs not only new music, but only a select few of my older, already synced music

When I connect my ipod to my computer, Itunes automatically launches like I set it up to do. Then, any new music that I've put onto my computer, syncs with my ipod. This is all fine. However, I notice that some of my music seems to sync EVERY TIME I connect. It's the same cds that do it over and over again. My ipod doesn't have duplicate copies as a result, and I dont' see anything duplicating in my library on the computer. So it's not really a problem, but more of a WTH is going on thing. Any suggestions ?