The symmetry between measuring voltage with the proper then reversed polarity used to be a spec listed on DMMs through the 1960s to early 1980s. By then, IC instrument amplifiers pretty much removed any practical difference, in robrenz listing, that different is ~ 1ppm, within his cal specs very likely for just the right polarity.
I tested the HP 3456a using the EDC MV106 as a source at 10V, 1V and 100mV, the difference is also 1 ppm or less.