The less available air, the most resistance, the harder the motors have to push. And why would there not be enough air? Simple: they're inside a case. You don't have more than a few cubic feet and things like fan grills and smaller than ideal vent holes are all reasons for noise. Furthermore, turbulence is also created with anything obstructing the airflow close to the fan, i.e. finger guards/case fan cutouts. So it's not exactly 20 and 21dba, respectively. Add 1-3 to each for real world results. On a side note, it would be interesting to see 38mm versions, I bet they could pump even more air while still being nearly silent.