the headphone output will always deliver the Master Stereo output source.
As Ruefus already stated the difference could be caused by the Space effect that is activated for the headphones by default. You can either activate it for all outputs or turn it down for the headphone output if you find that it is adding too much ambiance.
If you still encounter the issue please try a different pair of headphones or if you use an adapter try a different one. Maybe the one you are currently using is causing some phase issues which can potentially cause a misbalance between the dry and wet signal ratio.
Ok, I will try a different pair of head phones. Thanks