4.2BSD System Manual specifies it as mmap(addr, len, prot, share, fd, pos). There are six arguments. My best guess is something went horribly wrong with off_t on big endian systems and that it somehow leaked into x86_64 system call abi. Do you know?

off_t needs 64bit alignment so registers get copied to the stack argument structure correctly on kernel entry. This is ancient voodoo.