For the summer circuit, did you intend R1 & R3 to be different values? This causes different weighting factors for your two voltages which you are comparing.
Another thing that can make a difference: VDC2 needs to be able to sink current coming from VDC1. If it is not able to, then you can't be sure of getting a correct summed voltage at the non-inverting input of your op amp.
Are there unseen/unknown resistances upstream from R1 & R3? Diodes, transistors, etc.? Something that might help is to install a resistor at the non-inverting input of the op amp.