Sql-server – Why can’t I shrink the logfile below to the requested value

Experimenting VLFs behaviors with my test lab. Firsty, I have created two databases (BASE2 & BASE3) with different log files size (1024 & 2048 MO) :

Secondly, I'am running the following queries to determine log space usage and the active portion of the VLF :

USE BASE2
dbcc loginfo
GO
USE BASE3
dbcc loginfo
GO

DBCC SQLPERF(LOGSPACE)

So in my opinion there is plenty of space that I can reclaim. Now starting this query to shrink all databases log files :

DECLARE
@db nvarchar(255),
@fileName nvarchar(255),
@cmd nvarchar(255)

BEGIN

DECLARE dbcursor CURSOR
FOR

SELECT table2.name AS BASE, table1.name AS FICHIER_JOURNAL_BASE

    --table1.database_id, 
    --table1.physical_name, 
    --table1.type_desc
FROM sys.master_files AS table1

INNER JOIN sys.databases AS table2 
ON table1.database_id = table2.database_id

WHERE table1.database_id > 4 AND table1.type_desc = 'LOG' 
AND table2.state_desc = 'ONLINE' AND table2.recovery_model_desc = 'FULL';

OPEN dbcursor  
FETCH NEXT FROM dbcursor INTO @db, @fileName;
WHILE (@@FETCH_STATUS <> -1)
BEGIN

--print @db;

 SET @cmd = 'USE ['+ @db +']; DBCC SHRINKFILE ('''+@fileName+''',512);'
      PRINT 'USE ['+ @db +']; DBCC SHRINKFILE ('''+@fileName+''',512);'
        EXECUTE (@cmd)

FETCH NEXT FROM dbcursor INTO @db, @fileName;
END
CLOSE dbcursor;
DEALLOCATE dbcursor;
END

end here the result :

The requested value was 512MO but the current value on disk is around 650MO. If I restart the query with

USE [BASE2]; DBCC SHRINKFILE ('BASE2_log',8);
USE [BASE3]; DBCC SHRINKFILE ('BASE3_log',8);

the value on disk will be around 250 MO. So why my firt query did not shrink the log files to the requested value 512 MO ? Indeed, There was no active VLF at the end of file and still a lot of unused space.