The Data Precision 8200 is very temperature sensitive. You also need to make sure that the relays are clean, if not, the measured value will be erratic. Set it for 10V and lightly tap the relays, the reading may change a bit, but should return almost exactly. Same at 100V. If not, you can remove the relay covers and clean the contacts.
The 68K resistor in the output amp is undersized and often bad, I always replace them.
Sometimes the 100 MV divider has drifted OOT, and must be trimmed very slightly to get into spec.
You should not use nickel plated leads for the measurement, use copper due to thermals.
I made an adjustment cover that has holes to access the pots, leaving the cover on. You need at least a 7-1/2 digit dmm to make the adjustments, I have always used and 8-1/2; ie, 1281, 7081, 3458A..
Make an adjustment, let it sit awhile; check, adjust, repeat. You may need a few passes of the entire adjustment procedure.
Sometimes the resistors have drifted enough that an adjustment won't have enough range. Then you need to very slightly trim something.
These are a decent compact DC calibrator within their limitations.