Nothing to be confused about.
That's probably what can be expected considering your cases airflow.
If you case was big enough to fit a D-14 or Silver Arrow their performance would still hampered by case airflow dynamics.
You can expect much better results in a case with 2-3 x 120mm fans.
From looking at this it appears to me you can add a 3rd fan on the side panel.
I would also remove the vent port on the side panel and leave it open.http://www.giga-zone.com/tw/products/features.php?prodNo=50050437