What process are you using to determine what amount of RAM the SQL Server instance is using?

The maximum server memory is only for what the buffer pool will allocate. At the other end the minimum setting is to specify when SQL Server has to release buffer pool memory it should not go below that value.

If you are using Task Manager to view the amount of RAM the "sqlservr.exe" process is using for your instance that is showing RAM for more than just the buffer pool.