Comment by jmgao

They were renumbered in x86_64 so that the syscalls that are frequently used together have their function pointers live in the same cacheline in the lookup table: https://lkml.iu.edu/hypermail/linux/kernel/0104.0/0547.html

I vaguely remember reading somewhere that the MIPS ones are weird to support compatibility with the existing unix syscall numbering, but I can't find any evidence for that anywhere, so maybe it was aspirational or I'm hallucinating.