you are simply adding two voltage sources in series, as you can note on its spec. there is 0° phase between them not 180. 180° would cancel each other out you would get 0 volts. You see?
Your oscilloscope is connected wrong, cause the under source is connected to the oscilloscope in the contrary polarity... so it seems the sources are 180° but they are not.
You see that clearly in the oscope, you get chA+16ch B-16 summing=0 , you are only getting this waveform cause you are seeing two isolated tracks... if you add them up on the scope, this you will show you 0 volts. for this exact configuration.
This phase angle is relative to a source...
BR
Sebs