The natural variations in the diameter of the cane mean that, though the blade will make an even gouge along the bed which the cane sits in, it may not make an even thickness from a piece that does not exactly conform to the bed’s diameter.
Think about it this way, the blade and the bed of the gouging machine are fixed. When the blade moves along the bed, it does so at a specific distance above it. Now put a piece of cane which sits well in the bed at one end, and doesn’t quite at the other. After gouging that piece, you’ll have one side close to the thickness you would like, and the other not.