alt Hacker News> all the inputs and outputs share the same ground, it's not just the values for that pair of wires?
No, it depends on the converter. There are converters that leave 160V on the DC power rail for a 110V AC input, and 155V on the DC "ground" rail.
They are economic and you could find then when galvanic isolation is at least in theory not important, but they're terribly unsafe when used on PCBs that people might muck with.
If you have some "normal" converters and some of this kind, sharing the ground would be quite dangerous.
I have done some projects that needed some generic dc-dc converters from aliexpress (eg stepping down 12v to 5 or 3.3) I alway treated the output of each step down as a pair of wires that share no ground. It sounds like that would be overkill if they were reputable but it's probably best to not try tying the grounds together.
I figured any happenstance from the multimeter that the grounds match was transitory and not to be trusted.