Here I am slightly older and slightly "smaarter". I attended a system tuning training by Rational Acoustics and was able to get an answer to my question!
Mix coherency refers to the ratio of change between a measurement and reference signal. When tuning a system you would typically use software like SMAART or SysTune to generate a test signal (pink noise). This pink noise is sent out of your computers DAC, fed through the system, and out the PA. At this point is is picked back up by a measurement microphone placed in the listening area. The measurement signal is also immediately routed back in to your DAC. SMAART will compare this reference signal to the signal picked up by the microphone. If they are an exact match then you have 100% coherency!
However, there are many system and environmental factors that will affect the coherency. Is there a large HVAC hum at 200hz in the room you are measuring in? Your measurement mic may pic this up and show that your system has a large bump at 200hz. Once compared to the reference signal the measurement software will see the difference and will lower the coherency rating.
This is useful for making level / eq decisions. A 200hz bump @ 100% coherency means that there is something making that boost in the system. A 200hz boost @ 50% coherency means that it is probably something outside the system that is causing the boost.