Why the "swap used" is 1.4GB even I have 5.5GB RAM free??

I have a macbook air 13" with 8GB RAM, and by checking in Activity Monitor, the "swap used" is 1.4GB, even there are 5.5GB free RAM!

As I know swap on SSD is not a good thing, so how can I reduce the "swap used"? It seems the reason is not lack of RAM. I only have a few application, like mac office, adobe reader, and parallel desktop running Windows 7

Anyone could help?? Many thanks!🙂