ground is the 0V reference, it carries the unbalanced portion of a load to ground (or am I getting confused with AC and common)
but if channel 1 was set to 5V and channel 2 was set to 10V, sure my output reading would be 15V across both, but between ground there is an unbalance where one rail is 10V and the other is 5V
I've measured, and get some differences I'd consider significant, but I'm not sure if it's my meter that's on it's way out, or if the PSU is that far out.
yesterday I had it set to 5V and was measuring 5.4, so I turned it down until my meter read 5V and then the PSU said 4.6.
that's what I figured, I wonder if there is a way to adjust the actual output. Once I calibrate the displays, then it may not reach 30V on the upper end.
I see three trimpots on the PSU schematic. One of them is to adjust the max range of the PSU voltage output. At this point I would have to analyze the circuit in order to tell you which one to adjust.