This is because the stereo effects are leaking from the Left to the Right channel & visa versa. This is normal in most synthesisers with effects. To realize a mono output you will have to turn the stereo effects on the desired sound to OFF. To get a sound out of output 3 only for example, select the part then set the part outputs parameter on the output menu to 3&4. Now turn the panpot fully anticlockwise and turn the Delay, Reverb & Chorus effect sends to zero and the sound will only come out of output 3