MongoDB ACKNOWLEDGED write concern faster than UNACKNOWLEDGED?

I've got a very simple test program that performs faster with ACKNOWLEDGED bulk inserts than with UNACKNOWLEDGED. My real application exhibits this far more dramatically than this test program - about a 15x factor!

My understanding of the difference between these two write concerns is solely that with ACKNOWLEDGED the client waits for confirmation from the server that the operation has been executed (but not necessarily made durable), while with UNACKNOWLEDGED the client only knows that the request made it out onto the wire. So it would seem preposterous that the former could actually perform at a higher speed, yet that's what I'm seeing.

I'm using the Java driver (v2.12.0) with Oracle's Java JDK v1.7.0_71, and mongo version 3.0.0 on 64-bit Windows 7. I'm running mongod, completely out-of-the-box (fresh install), no sharding or anything. And before each test I ensure that the collection is empty and has no non-default indexes.

I would appreciate any insight into why I'm consistently seeing the opposite of what I'd expect.

Thanks.

Here's my code:

package test;
import com.mongodb.BasicDBObject;
import com.mongodb.BulkWriteOperation;
import com.mongodb.BulkWriteResult;
import com.mongodb.DBCollection;
import com.mongodb.DBObject;
import com.mongodb.MongoClient;
import com.mongodb.ServerAddress;
import com.mongodb.WriteConcern;

import java.util.Arrays;

public class Test<T extends Number> {

  private static final int BATCHES = 100;
  private static final int BATCH_SIZE = 1000;
  private static final int COUNT = BATCHES * BATCH_SIZE;

  public static void main(String[] argv) throws Exception {
    DBCollection coll = new MongoClient(new ServerAddress()).getDB("test").getCollection("test");
    for (String wcName : Arrays.asList("UNACKNOWLEDGED", "ACKNOWLEDGED")) {
      WriteConcern wc = (WriteConcern) WriteConcern.class.getField(wcName).get(null);
      coll.dropIndexes();
      coll.remove(new BasicDBObject());
      long start = System.currentTimeMillis();
      BulkWriteOperation bulkOp = coll.initializeUnorderedBulkOperation();
      for (int i = 1; i < COUNT; i++) {
        DBObject doc = new BasicDBObject().append("int", i).append("string", Integer.toString(i));
        bulkOp.insert(doc);
        if (i % BATCH_SIZE == 0) {
          BulkWriteResult results = bulkOp.execute(WriteConcern.ACKNOWLEDGED);
          if (results.getInsertedCount() != 1000) {
            throw new RuntimeException("Bogus insert count: " + results.getInsertedCount());
          }
          bulkOp = coll.initializeUnorderedBulkOperation();
        }
      }
      long time = System.currentTimeMillis() - start;
      double rate = COUNT / (time / 1000.0);
      System.out.printf("%s[w=%s,j=%s]: Inserted %d documents in %s @ %f/sec\n",
          wcName, wc.getW(), wc.getJ(), COUNT, duration(time), rate);
    }
  }

  private static String duration(long msec) {
    return String.format("%d:%02d:%02d.%03d",
        msec / (60 * 60 * 1000),
        (msec % (60 * 60 * 1000)) / (60 * 1000),
        (msec % (60 * 1000)) / 1000,
        msec % 1000);
  }
}

And here's typical output:

UNACKNOWLEDGED[w=0,j=false]: Inserted 100000 documents in 0:00:01.021 @ 97943.192948/sec
ACKNOWLEDGED[w=1,j=false]: Inserted 100000 documents in 0:00:00.909 @ 110011.001100/sec