You keep insisting a DMM6500 is a low grade instrument but other than from the perspective of a rabid volt nut that seems to be pushing it. 0.0075% and that dynamic range is pretty good in the real world and bought for other reasons than say a Fluke 87V.
DM6500, Keysight 34460, 34461, 34465 and all similar instruments are not metrology grade instruments. Sorry to disappoint.
They all come with datasheets that exactly specify what are their measurement uncertainties (that are in several tens of ppm in best ranges) . Even if you have calibration data that shows that you instrument had zero error on say 10V range three months ago, today, all anybody knows is that you should be in 90 days accuracy spec that for DMM6500 would be 20ppm + 5ppm ±(% OF READING + % OF RANGE) ), meaning 10 V ± 250uV. You might be better than that (and probably would be) but if you had even 248 uV of error (meaning last 2 digits would be wrong) you still would be in specification and instrument would be considered in good shape. On other ranges situation is much worse actually. Keysight 34460 that is also 6.5 digit meter, is much worse on 10 V range. It has 90 days spec of (50 ppm + 5ppm ±(% OF READING + % OF RANGE) ) that translates to error of max ±550 uV on 10 V voltage, before it is out of spec. More than half of millivolt.. And that is for 90 days. More than 1 years after calibration, 34460 can have ±1 mV error when measuring 10V and still would be in spec. That would mean, that at that point, last 2 digits are completely meaningless in terms of absolute accuracy..
Most of instruments in that class do much better than spec, but manufacturer guarantees only that they will be in spec. And if you use those instruments in your work, all you can guarantee to your customers is that you measured with instrument that is in spec..
That is for absolute accuracy measurements. For relative measurements, all you can guarantee is 24 hour specs.
So when you accept those facts, why would manufacturer give you data that is meaningless, except statement instrument is in specification... If you want play with instrument pretending it is something it isn't, that is users problem. But, even if you calibrate DMM6500 every day and it has no error for 5 years, and suddenly one day it has 13ppm error on 10V range, it would still be fully in spec.
Actually, compared with competition, DMM6500 has very impressive specs that almost seem too optimistic. It has better 2 year specs than Keysight 34465 1 year's spec on 10 V range for instance..
But none of those are Keysight 3458, or Fluke 8508A, and calibration practice is not the same...