I think if what @dlang posted has happened (right chain skipped a link or two), then if you cut a straight line across the board, it would possibly stay straight, but would end up lower at the right side. You can test that by moving the sled to -42,0 (6 inches from left side) and make a mark where the router is, move to 0,0 and make a mark, then move to 42,0 and make another mark. If the three points form a line that slopes down from left to right, then I think that indicates a skipped link with the right chain. If you find more of a parabola, then I think it’s more a calibration error.