It would be a nice thing to know the initial values.
From the "more serious" Keysight I know them, because they were in the form of a protocol with the delivery, since then it has been calibrated regularly(so you can see where the values drift over time).
From my serious Siglent, I only have a sheet of paper....
I will write to them, the serial number is individual, maybe there is a database with values.
If you have your gear calibrated by some independent cal lab, you'll have to pay more for a calibration certificate with data than one without. Considering the price of a comparable Keysight, it appears quite appropriate if they provide a calibration report with data.
Even with data, you don't gain as much as you might expect. The data are just a snapshot of the instrument under certain test conditions in a certain environment at a certain temperature, taken with some references that have a certain tolerance which should be guaranteed by a traceable calibration.
By contrast, you use the instrument in a different setup, under different conditions, at a different temperature and some time has passed since the last calibration. I can tell from experience, that you cannot ignore this and especially a brand new instrument may drift significantly (yet staying within its one year specification) during the first months of duty, so the cal data would not be terribly useful for introducing "correction factors" anyway.
This still describes the ideal case, when the calibration is done carefully by well trained and experienced personell. While this should be the norm, unfortunately this isn't always the case.
In short, if an instrument has a certain specified tolerance, then you can have some confidence that the measurement results will be within this tolerance window - if you use an appropriate test setup, that is. Your own table above demonstrates how even fairly accurate instruments can deliver just house numbers under certain conditions. A calibration certificate with data might tell you that the meter was off by some 5 ppm in the 20 V range when tested in the cal lab. What would this info have helped in this very situation, where the reading is off by several hundred ppm?