the 57xx do have *relatively* uncertainty spec,
if one have better cal standards than fluke have to calibrate 57xx,
then the *abosulte* uncertainty of 57xx could be calculated as new spec, better than fluke published one.
but the published *relative* spec contains many things...it is defined for long-term behavier.
i think 2ppm is the uncertainty transfered from NIST 10v to keysight 10v (NIST 732 to KS 732?? maybe),
but anyway 3458 spec has a lot of head room, if one has years of calibration history of 3458, i think it is common that it is not out of 1-year spec for years, and so is fluke 8508 i think.
so a good calibration and history is really helpful for one to estimate a better spec rather than published 1-year spec, that calibration make sense.
with lower or higher cal uncertainty it is for one to trust the reported cal data, more or less. normally TUR is kept below 4:1 or 3:1 or use transfer technique, like a specially calibrated 57xx to calibrate a 3458, the drift and repeatability is thoroughly verified at *short* period, e.g. every 30days.