The packages aren't automatically upgraded to the SQL Server 2012 package format.
Per BOL -
"At runtime DTEXEC uses temporary upgrade process for an existing SQL Server 2005 or 2008 package that may increase execution time."
More information available via:
http://www.sqlservercentral.com/blogs/dknight/2012/05/16/upgrading-packages-to-ssis-2012/[/url]
http://msdn.microsoft.com/en-us/library/cc280546.aspx