From UVLayout Docs

Here's one example of the use of the Rescale tool. The top row of images shows some hair modeled as ribbons, and a layout created using the Stack tool. You can see that some ribbons are red and some blue because they are different lengths in 3D but have been forced into the same length in UV when stacked.

If this distortion is a problem, it can be fixed with the Rescale tool. If you click on the & button, each ribbon will be scaled in both the U and V directions, which in this example causes the longer ribbons to get fatter and overlap (see left image). If however you click on the V button, each ribbon will only get longer or shorter, preserving their spacing (see right image).