Dedicated server does not release/unlock missions files.
Example
You play mission A (server locks & loads it). After that mission you decide to play mission B. The mission A pbo is kept locked.
Unlocking it would make updating missions a lot easier, because you wouldn't need to stop server every time to be able to replace the mission file.
Same behaviour was in Arma 2: http://dev.withsix.com/issues/5583