I did a quick check on the Rigol DP832 and saw a review of a dutch customer mentioning a 30uA current and +-50V AC potential between the posts and ground.
I had to check this for my Siglent SPD3303X-E, and it was 420mV AC (@10 Mohm), and maybe 0,0420 uA of current.
What does it mean? In the Netherlands (and I guess many more countries) one of the mains wire (neutral) is connected to earths ground. If there is capacitive coupling between the primary and secondary windings then that can be measured between earths ground and one or both of the outputs of a channel.
This is not a problem until earths ground is added somewhere to a circuit (scope or signal generator or just earthing). When it does it, the AC voltage can then give unexpected results. I know because I had to debug a situation with a supply of my own making (70V AC @ 10Mohm). It's those toroidal core transformers that are prone to these issues. Primary and secondary windings are then relatively close to each other. With those transformers it is better (and more expensive) to have some shielding between primary and secondary.
To be honest I'm glad the Siglent seems to be OK regulated down. There will alway be some coupling, but there's a difference between 70V AC and 420 mV AC.
Each post has its own degree of coupling (different secondary winding):
1) 420 mV
2) 258 mV
3) 108 mV