The 40mm lens for the CL (not CLE) lenses had a more steeply sloped RF coupling cam than other M-series lenses. This wouldn't have made a difference on a perfectly-set-up camera, but a given amount of angular error would produce more focusing error with a CL lens than a regular lens. There was a lot of discussion of this back in the day, but most likely the forgiving DOF of a 40mm lens prevented it from being a real-world problem. I don’t remember if the 90mm cam was the same way or not.