I'm not exactly sure why, but I measured it, so I know it is happens.
Here is my explanation. May be right, may be wrong.
As you alter the third link length, it twists the axle housing, which changes the angle of the radius rod bracket on the bottom of the axle to lean toward, or away from the radius rod lollypop on the chassis. That changes the distance between the centerline of the axle and the lollypop. If you have different radius rod lengths or angles to begin with, it will effect each side a little differently. It is the same as changing radius rod lengths. When you change them, you are changing the arc of travel and effecting the rear steer of the car. I am VERY particular about knowing the squareness of my car's rear end.
How much or how little these things will change and the magnitude of their effect is something that I do not know how to calculate. I probably could if I really wanted to, but I'm still dealing with bigger fish with respect to setup. I do know, however, that my son can tell when I change the RR radius rod length by as little as 1/2 of a turn. You can see it on the clock and you hear about it when he comes off.
Thanks for the explanation Steve. Your info and insight has me questioning things, and that is what I need. I'm not going to learn a darn thing doing everything the same way over and over.