+1 for the likelihood of an asymmetrical setup and different first reflection path lengths for each speaker. Try increasing the toe in of the speakers at least temporarily to help reduce the amount of 1st side wall reflections. If more toe in = less difference between the speakers then consider room acoustical treatments or repositioning of speakers, or chair or both.
Have you measured the output up close to each speaker to see if they are outputting the same decibels?
Have you measured the output up close to each speaker to see if they are outputting the same decibels?