Sync Music to iPhone will erase all content?

So I've had this iPhone for a while, worked just fine. Now this weekend I was with a friend and wanted to play this music of an artist but found that I hadn't added it to my iPhone. So today I plugged in my iPhone, click on it, click on the Music tab, and for whatever reason the Sync Music box is not checked off, and by attempting to click it it warns me that syncing will cause all existing content on the iPhone to be erased then replaced. This despite the "Selected playlists, artists… " etc circle being selected beneath the sync music option, which is why I have several playlists from my computer synced to my iPhone.

What is the dealio?

It just means if you have any music that's on your iPhone but not your iTunes it's getting erased. I don't think it literally erases everything and reloads it, that would take an eternity. I'm not sure what triggers the warning but I always just ignore it.