I see people have taken into account the errors of the resistors but you also need to be wary of the effects that your meter has.
For example, the
specs talks about its accuracy specifications over 24hrs with a room temp kept within ±1°C. So a few of the ppm drift may well be because of the meter settling.
Even the test leads might have a bit of time to adjust.
Be very cautious of thermals.
But experiment, the more you look the deeper you go. Just don't end up like n00py ;-)