lucene merge indexes produces different results
===

I tried to test the results of merging two indexes with IndexWriter.addIndexes().
I did this steps:
First I created only one index with the whole dataset.
Then I split the dataset in two parts and I create an Index for each part,and finally merge the two subindexes with the code below.
Then I use mahout's lucene.vector driver for the two indexes (the first from the whole dataset and the merged).
So, I see that the two dictionaries (one for each index) that are produced from mahout have small differences but it should be the same.
Could anyone tell me why is this happening?

    Version VERSION = Version.LUCENE_36;

        File INDEXES_DIR  = new File( args[0] );
        File INDEX_DIR    = new File( args[1] );
        
        INDEX_DIR.mkdir();

        IndexWriterConfig config = new IndexWriterConfig( VERSION, new StandardAnalyzer( VERSION ) );

        IndexWriter writer = new IndexWriter( FSDirectory.open( INDEX_DIR ), config );

        Directory indexes[] = new Directory[INDEXES_DIR.list().length];

        for(int i = 0; i < INDEXES_DIR.list().length; i++)
        {  
            indexes[i] = FSDirectory.open( new File( INDEXES_DIR.getAbsolutePath() + "/" + INDEXES_DIR.list()[i] ) );
        }
        
        System.out.print( "Merging added indexes..." );
        writer.addIndexes( indexes );
        System.out.println( "done" );

        System.out.print( "Optimizing index..." );
        writer.optimize();
        writer.close();
        System.out.println( "done" );

  