Introduction SharePoint 2013 introduces a new model on how Site Collections are upgraded using the new Deferred Site Collection Upgrade. To keep it simple it means that SharePoint 2013 can run a Site Collection in either 2010 mode or 2013 mode, and SharePoint 2013 contains a lot of the SharePoint 2010 artifacts (JS files, Features, Site Definitions) to handle this. When you’re doing a content database attach to upgrade from 2010 to 2013, only the database schema is upgraded and not the actual sites (by default). The actual Site Collection upgrade is done by the Site Collection administrator when they feel that they are ready to do that and have verified the functionality of SharePoint 2013 (or you force them to upgrade anyways). But, the Site Collection admin might have to upgrade sooner than expected for some sites.