The compiler/linker writers associated with a language design committee usually impose some limits so that they can make assumptions in the implementation of the toolchain or in the accompanying binary file formats (e.g., ELF, COFF, etc). The numbers 31 and 63 were probably chosen simply because they are 2n-1 and programmers like 2n for some silly reason. The -1 is usually to account for either a leading _ or a trailing NUL character in the name.
