Check the manual for the DMM in question. It should say what kind of accuracy you can expect.
One of my HP's have a "High Resolution" mode, which sacrifices update rate and takes a lot of samples and averages them out which sometimes gives you an extra digit depending on your range.
They might mean something like that, but again, that would be mentioned in the manual.
If you're measuring a clean supply, you get a clean reading.
If your supply is full of ripple, it doesn't matter how good the multimeter is, you're still gonna get the last digit(s) jumping around.