The first is the ADC quantization. In the PowerXR products the LSB (Least Significant Bit) of the ADC is 5mV, so each one of these lines represents 5mV.  If the target value is somewhere in between, users will regulate at some point on either side of that target and it will move. Quantization will cause it to regulate at one point and another point. What will result in is a small amount of ripple on the output.