Large Gain Difference Between L and R Speaker

I just measured my FOCAL Twin6 speakers and found a 6.5dB gain difference between the L and Right speakers. The speakers are centered along a wall and room treatment is symmetrical. I'm using a brand new UA apollo x8 2nd generation audio interface.

I adjusted the LF and HF Levels on the back of the Twin6 speakers as much as possible; +3dB on the L, and -3dB on the R, and repeated the measurement. This time, there's a 4.7dB gain difference between the speakers. I would have expected a 6dB swing, not 1.8dB.

How important is it that the gain differences be minimal? Assuming it's unusual to have a difference this large, where do I start troubleshooting? With FOCAL? With UAD? With Sonarworks?

Thanks,
Tom