Just sharing an experiment... There are awesome Voltage References as "DMMCheck Plus" and "PDVS2mini 20bit", and also cheaper options like AD584 and LM399.
I got an LM399 on Aliexpress, I was waiting for a good product and (very important) a Precise Calibration. I tested a very new Fluke 289, Gossen 29S, Richmeters RM303, and Aneng AN870.
None of them exactly matched the LM399, but the difference is between 0.02% and 0.01% (or 1/10,000), an excellent precision!!!
I'm happy that Fluke and Gossen exactly matched ALL the measurements (both have a Calibration certificate and they scored exactly 5.0000 VDC). Also, the chinese DMMs performed Very Well.
I think a factory produces a bunch of LM399 references and each seller is responsible to manually calibrate it. I saw some pics with the Voltages printed, handwritten, or even photocopy. Maybe (maybe) the seller uses a regular DMM and "create a fake decimal value", like 5.000V "un-rounded" to 4.99955V.
Also, even with a stable 15V input and constant 24C temperature, there is a voltage fluctuation of +- 0.1mV.
In summary: get a good quality Voltage Reference, Correctly Calibrated by a Trustable seller, unfortunately, on Aliexpress not only the products and specs can be fake, but also the Services. Now, I need to find somewhere a +6 1/2 digits DMM to check the correct calibration.