Presenting evidence that this cannot be right (see images), the response was:
I'm not quite sure if I understand you.
I believe the DPOX180H simply displays the difference between the dashed lines in relation to the respective vertical sensitivity (V1 Ch#1, V2 Ch#2).
It doesn't show absolute levels afaics (Like Y1, Y2 with the Zeeweii).
This is strange, but not directly a bug imo.
The DSO2512G displays the two absolute values Y1, Y2
and the difference dY in relation to the vertical sensitivity setting of the selected channel.
If you want to see the values in relation to the vertical sensitivity setting of the second channel, you can switch the channel priority. The values are then displayed in green.