Scopes have their best accuracy towards the high end of its scale, as you've pictured. But it can vary depending on vertical gain too, from issues with the vertical amplifier, independent of the amps frequency response. The published figures should be worse case, and at least, Rigol 1052e does live up to its spec sheet, at my last evaluation.
Try a variable PSU, adjust the voltage down while watching the output with your DMM, and compare it to the Vout reading of the Rigol. When its as small as possible but still measurable, adjust the Rigol's vertical gain to bring that DCV to full scale, and compare again.
Changes in environmental conditions can cause offsets too; for best accuracy before use, at least on the Rigol, do a self-cal before use and insure the scope is warmed up 30+min.
Just as a data point, I did a quick check on my Rigol with a 5 V reference source. As you can see it settled at the right voltage. (It read the same on Ch 2 as well.)
However, on this vertical scale the precision is 0.04 V so waving my hand near the probe could make the voltage jump around between 4.96 V and 5.04 V. Since full scale is 8 V, that would give an accuracy of about +/- 0.04/8 = +/- 0.5% on DC at this scale.