The right side is further away from the side mirror. This means that any variance in anything, the left side would be the first to cause issues.
You say it has happened since day one. Where is your GlowForge located? Is the air humid or dry, hot or cold, dusty or clear? Is this in a garage or a living space?
Also, if there is an alignment issue, the further you travel away the worse the alignment issue will become. Have you tried pushing it back and seeing if there is any extra 'play on the right side compared to the left? If you do this, make sure you do this gently, slowly and while the machine is off.
I might also cut out eflyguys focus height ruler and make sure that the ‘printer head’ is at the same level between left and right side.