Peter,
The problem is one of simple configuration of the Install / Update manager. If you navigate to Window > Preferences > Install/Update, you’ll see a setting for ‘Valid Updates’. You need to set this to ‘compatible’ in order to see 2.7.0 GA since it’s a minor update over 2.6 and the RC builds. Once you do that then research, you’ll be able to see the update correctly in the Install/Updates perspectives Preview view.