Why do I only have a capacity of 13.5 GB when I have a 16GB iPhone 4s?

I keep trying to download new music (only about 10-20 new songs), and it keeps giving me a message that I have to have at least 5 free GB to successfully sync the music to my iPhone. I've never had this problem before and I've actually gained more storage space since the last time I synced my phone, so why is it sending me this message?