Thanks for the tips all.
I re-examined the gantry and on second pass I found some very slightly loose wheels that I tightened by turning the eccentric nut. After making this fix, however, I am still seeing the same result with a narrower slot than I would expect.
I’ll look into calibrating the machine, and I’ll also check into the suggestion that @RichardLeather made about toolpath in/on/out boundary.
This time I ran the machine and after the 3rd pass the tool doesn’t touch the wall of the slot on the finishing pass, which leads me to believe this could be a CAD problem. It seems the tool path is moving the machine to the outer most point that it has in g-code, but this is shy of the final intended width.
Also, FWIW the slot is not narrow only in one direction on the x axis. When making a 2nd pass of the same cut, the bit cuts material on both walls of the slot, meaning that the machine is at least not offset in one direction on the x axis.
To say that another way, the slot is too narrow symmetrically from the center line of the slot.