The BM857 has over 50 calibration points, for example, covering every function and range. So one photo at 5V is not showing much of anything.
FWIW my BM857s and Fluke 187 agree within a digit or so on every range except small currents*.
Both meters have laser trimmed precision resistor arrays which basically don't drift. All measurements are derived internally from voltage measurements so if the voltage ranges are good then the rest of the meter probably is, too.
There is absolutely plenty of opportunity for drift. Checking one point is just not a valid method for this. If those two DMMs haven't drifted, then that is great, but what you're saying is not something that can be applied and the photo isn't proof of the method.
Some DMMs DO follow a principle adjacent to this where they know which components are leveraged for each measurement path. So a performance verification procedure will heavily leverage that in order to reduce the number of tests that need to be performed by the operator.
The protection circuits can also make an impact depending on the voltage you are applying, so each range will still need to be checked.
Edit:
Link to the Fluke 187 calibration guide:
https://www.tonyplaza.nl/download/YT251/187cal.pdfThere are 64 performance verification test steps.
There are 38 calibration inputs required.