We were measuring our DC levels in a high-electrical-noise environment
(> 20kV, fast risetimes).
All lesser digital voltmeters (and often our digital o'scope)
would go crazy and/or give outrageous readings
when the ambient noise would reach a level
where their microprocessors could not tolerate it (my best guess).
The only things that worked reliable were analog meters
and OLD John Fluke DMMs (newer Fluke handhelds had problems too).