DMM and Power Supply
Two things to consider:

An unregulated power supply will typically supply a higher voltage than is stated for loads which are less than its capacity. For example, if your unregulated power supply is 3.3V at 500mA, connecting a volt meter to it may result in much higher voltage readings because the meter is an extremely small load.

The values you give in your question, however, are more typical of normal precision variances in measurement gear. The accuracy of the power supply, and your meter, are probably not better than 1 to 5%. If you need greater accuracy, be prepared to allocate substantial funds to acquire a high-precision meter.

Edit:

According to your comments, you are using a high-precision meter and power supply, the Agilent 34970A Data Acquisition/Logger Switch and E3631A Power Supply.

According to the specifications for these units:


The E3631A Power Supply has a load and line regulation of &lt; 0.01% + 2mV. (Datasheet)
The E34970A Data Acquisition/Logger Switch has an accuracy in the DC 10 V range expressed as: $$\pm(\% \text{of reading} + \% \text{of range})$$ The value given for 24-hour 23°C is 0.0015 + 0.0004. (Meaning the unit has been calibrated in the last 24 hours and is operating at 23°C \$\pm\$1°.) (Datasheet)


If I am reading the datasheets correctly, these units are very precise. Getting a value of +20mV seems like a calibration problem.