slugs

We bought a used US Yacht 22 and need to replace a couple of the sail slugs. We found four different size slugs being used on the main: Pacific Nylon Plastic PNP 29 which is 9mm, and Aquabatten sliders 017S = 5/16", 018S = 7/16" and 019S = 3/8". My question: why would there be different...