Licenses in my account

Since November 2006 I have automatic renewal for my Norton product on my computer.  When looking at my account, I see 2 seats used out of 3 (one since November 2008 and the other one since May 2010).

I do not understand why I have the 2 licenses and why there would have been an installation done in May.  How do I know which one is valid?  I do not want to remove something that leaves me unprotected.  I only have the one computer.