MACHTYPE environment variable on ARM

I am on an M3 Pro, MacOS 14.2. When I try echo $MACHTYPE (in a default -zsh shell) the output is x86_64, whereas uname -mp returns arm64 arm. Zsh docs say that MACHTYPE should be "the machine type (microprocessor class or machine model), as determined at compile time." Why is my compile time machine type set to x86_64? I am not running Terminal with Rosetta. Perhaps a more robust question, what is the intended usage of the MACHTYPE shell parameter on ARM-based MacOS?