I just did some reading on the subject: Apparently, only inverting configurations can be used as mixers to properly sum voltages, which, I guess, is necessary in this case to add/subtract an offset from the DAC-Voltage. It is then fed into the variable gain Amplifier, and then again inverted to give the correct output-polarity. Actually not a bad idea, but in this case not up to the task.
What (I think) they should have done: Mix the signals with the first inverting amp, invert the signal again (to have correct polarity), feed it into the variable gain amplifier and then drive an amplifier configured as a non-inverting amp.
Would have added two ICs (or one if one doesn't care about crosstalk too much^^) to the BOM, plus some passives, and required some more space on the PCB.
I guess, this is how they do it on the High-Power Option that is available for these units, which fits in the large empty space next to the PCB inside the case.