It appears that the cannons are capable of firing closer than the graphic indicates.

https://i.imgur.com/a5uuUwc.png

I don't think the splash damage can explain the entire problem I'm seeing since I also note that shots are occurring further right/left of the defined segment as well. Am I wrong?

Are you talking about the cannon shells hitting biters closer than the minimum range?
First of all, the fire arc displays what the turret will "aim" as target. It is not necessary to be the same area the shells can deal damage to.
When a cannon shell is flying towards target, there are collision checks on the path. HE shells have low penetration power so can damage biters before reaching target.
So the minimum range is to prevent the turret aiming at biters already at point-blank range, like the suggested range of a (real-world) sniper rifle. But if you put you palm in front of the rifle barrel, it does mean your hand is safe when your pull the trigger.

For the left/right issue, I am not sure if you do not have clear examples.
In your screenshot, it is possible the corpses are where the left cannon turret can kill.

I was referring to the wall damage from nearby impact. I hadn't factored in collision versus point of aim. Nothing further, thanks!