Internal Vss digital ground not zero

Hi everyone,

I was going through the BusVoltage arduino example, and I found that my Vss reading is never 0 - around 3 and 4 - I was wondering if this is normal. 
I went to see the configuration of the ADC and it seems that its the right channel and all. 
Also, is it the ADC value, or the converted to voltage value? Because if it is in Volts that would be a weird reading.

Thank you