The Dosimetry Service applies the legal recommendations for dose calculation in Switzerland. The 2004 recommendation (original pdf document only available in German) by the Swiss Federal Commission on Radiation Protection states that all monthly doses below 75 µSv (=0.075 mSv) should be rounded to 0 when transmitted to the Authorities. Why is this so? At CERN we use a very accurate dosimeter: the DIS-1. It has a detection limit as low as 1 µSv, whereas most of the other dosimetry services in Switzerland use other detection techniques that have a detection limit of 50 µSv. Below this threshold they cannot tell if there is a dose or not. It is therefore understandable that CERN does not report doses that the other dosimeters cannot 'see'.
The second reason is that there is high fluctuation in background in Switzerland, depending on your location. If someone is in a region with a high background he will record a higher monthly dose than in low background regions. But this dose is the result of natural radiation and it should not be taken into account as a real dose for workers. This is why the experts have fixed the limit at 75 µSv. In practice, if you read a dose below 0.08 mSv on the reader your official monthly dose will be 0 mSv.