To imagine how this happens, look at the housing of the CV that you have been calling the cage (the cage is actually the part that of the axle that mounts on the splined axle shaft, that you snap the balls into). The housing is the part that has those long grooves in it that the balls ride in. In a perfect world, the balls would ride in the middle of the grooves. If you measure from middle, that only gives you, what, 3/4" of travel either direction? If your axle is too short, it hammers itself to death. If your axle is too long, it overextends and falls apart. But, there has to be some (axial) movement in the system because the axle needs to be length self adjusting. If the length is correct, these axles rock-if too long or too short they are maddening.