Yes, they are. But that's database version, not compatibility level.
A database on a SQL 2008 server is a SQL 2008 database with the SQL 2008-style statistics, the SQL 2008 system tables and the SQL 2008 file structure. That cannot be changed. The compatibility level doesn't change any of that. All the compatibility level is change how the Query Engine interprets certain SQL constructs. Nothing else.
That's why a database cannot be moved to a lower version, because all of the internal structures are for the higher version and there's no way to convert them to a lower version..
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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