Unclear common-mode voltage specs in the DMS-20LCD volt-panel-meter
I see your dilemma. Firstly, yes, you are quite right that strictly-speaking, the common mode voltage on the diagram is 2.5 volts but you could also make an argument that it is 1.667 volts with a signal of +1.667 volts on pin 11.

I think this is what they mean although by no means is this clear in their document. In other words, make sure pin 12 remains within +/-2 volts if you want accuracy.