Correct, it won't add up because the 2.8GB is not specific enough (you're looking below at data measured in MB). As you move more data over you'll see it change. Add 400MB of music and you'll see 3.2 to 3.3GB of space used.
There is the base storage used by the OS (around 2.8GB, but it will change over time) and then there is your specific data, listed below (of which you have almost nil).