Encontrei este link, Re: Mac OS X Snow Leopard e aplicativos de 64 bits que diz:
arch
anduname -p
both return the processor family type, i386. (as opposed to ppc or arm.) I think the rationale was that too many scripts depend upon this behavior.
uname -m
tells you which slice of xnu you booted from, i.e. x86_64 for the 64 bit kernel, i386 for the 32 bit kernel.It's unfortunate that the phrase i386 has two meanings.
Isso esclarece a questão.