Why doesn 1.3.1_03 work as a Java runtime platform version?
Platform vs. Product versions. Web Start differentiates between platform and product versions for Java runtimes. If you choose a vendor-specific product version such as 1.3.1_03 or 1.3.0_03 instead of a vendor-neutral platform version like 1.3, you always need to add a vendor-specific href (e.g. http://java.sun.com/products/autodl/j2se for Sun) or else Web Start assumes 1.3.1_03 is a vendor-neutral platform version and won’t find any matching Java runtimes.