In your measurements, you’ll get an L+R curve which corresponds to a mono signal send to both L and R loudspeakers. This L+R black curve is important because most recordings are mono in lower frequencies and it better represents low frequencies perception.
In some cases, the black curve is lower than each blue L and red R responses. This means that L and R signals are for some frequencies out of phase and instead of adding power, those signal destructing each other ! This has to be avoided and generally you will need to find better positions for your loudspeakers. With one subwoofer this cannot happen because summation is done electronically without any phase opposition. The hybrid correction computed by loudspeakers.audio tries to optimise L+R but sometimes it cannot completely solve this important problem.