I'm right handed, and my hearing loss is more severe in my left
I believe this is because the sound radiates from the muzzle, and the ear directly behind the gun is less exposed, due to the mass of the gun (particularly long gun) attenuating the blast, slightly. In other words, the non-shooting ear actually recieves more damaging decibels. The difference is slight, but over time, the effect is noticable.
I have an alarm clock that has a high piched chirp and a low pitched buzz. When sleeping on my back, or left side I hear the alarm normally. When sleeping with my right ear in the pillow, all I can hear is the low "brrrt, brrrrt, brrt," Which, by the way usually doesn't wake me up for nearly 20 minutes
My theory might be wrong, or it might only apply to me, personally, but it does explain why my left ear is worse than my right.