I’m not going to say that your Arc doesn’t have a manufacturing defect but is could be an optical illusion. I turned rotated my Arc 90 degrees (front facing upward) and placed a level in the center and both ends. The bubble was slightly off on both ends compared to the center measurement at dead center of the Sonos logo. Taking those measurements at face value I too would have concluded there was a curvature.
I decided to place the level directly on the surface of the entertainment cabinet upon which the Arc rested. The measurements were exactly the same. Conclusion either the cabinet needs adjusting or my foundation has a slight rise or decline (depending upon how one wants to interpret the findings).
In any event the unevenness is not in my Arc. Furthermore it’s not enough for concern as I would have never given it a thought if not for your post.
However, by all means contact Sonos if you feel you are seeing an actual curve in your Arc and it’s not an optical illusion.