Black magic pretty simple, it's tempco of all different parts inside meter. Yes, LM399 or LTZ1000 in meters is ovenized, but except voltage reference, there are resistors, capacitors, active parts all around measurement switches, function blocks, and all of them drift differently. It takes time to get all temps to stable level before you can get desired accuracy.
And instrument was calibrated on specific temperature, so you can see that as a single point on the temp graph. More off you from that sweet spot - less accuracy you have.
More precision gear just have that "in-spec" window smaller, so it's easier to miss 10ppm accuracy window on 8.5digit meter rather than 5000ppm on 4.5digit one
![Cheesy :D](https://www.eevblog.com/forum/Smileys/default/xcheesy.gif.pagespeed.ic.KPzuH4J-TG.png)
If you read carefully thru calibration manual on Model 2002, you can even note that presence of metal cover or scan card option affects calibration, so it's recommended to do calibration with
proper option installed
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