its how the virtual memory algoritm works (how it is programmed). The idea is that you always leave a bit of free RAM for new programs, and that that swap you see there is for programs that are running but are not often used. So, why is there someting in swap? To get more free RAM for more active programs, linux puts unused programs in swap. Why is there a little unused RAM? To make place with new programs (else, it would have to first make room for them and then load them).
My english writing is pretty bad at this time of the night, hope it could be understand =)