Can happen when you are not running the latest version i.e.
you are running 10.7.2 the app store has 10.7.3 so offers you to install it ...
we need to get used to this as its how we will see updates in ML
(If I download LN from the appstore it states also the 22 July 2011 but installs 10.7.3 as of today no need to install additional updates !!... this is the same from my 2011 Air, 2010 Pro, and 2011 Mini.... before some one picks up on the date )