The stability specification only guarantees that the outputs will not change by a certain amount over an 8 hour period with a constant load and environment. It does not say anything about absolute accuracy.
Absolute accuracy is determined by comparing the output of the power supply to some external calibration standard. The one year accuracy spec for the power supply says it won't deviate by more than a certain amount for one year from the last calibration.
If you are concerned about the absolute accuracy of the power supply, you can use a more precise instrument to monitor the output and adjust the power supply using those readings as you go or, use that instrument to adjust the internal calibration of the power supply as often as you want. The Rigol calibration guide for the DP800 series power supplies is attached below. To do the calibration, you would need a DMM that has the accuracy that you want. A 6 1/2 digit bench DMM is recommended, but you could even use one of the very inexpensive Chinese 10,000 count DMM's (e.g.
https://www.banggood.com/search/an8009.html) and use that as your calibration standard. This would get all of the channels of the power supply to have a consistent calibration but would only have the absolute accuracy of the DMM you are using as your standard. It's all relative. It depends on what you need the power supply to do for you.