I think that might be `git symbolic-ref` which doesn't work in detached HEAD state (e.g. when checking out a commit) whereas `git rev-parse --abbrev-ref` will print "HEAD". On the other hand, `git name-rev` will try to find the nearest named ref (including tags) and how far HEAD is from that named ref (e.g. master~1).
Internally, symbolic-ref resolve the ref in the same way as git branch, just without handling detached HEAD. __git_ps1 use a combination of rev-parse, symbolic-ref and git describe to provide a more interesting output for displaying state of the working directory.
To have the similar output for detached HEAD as git-status (e.g. "HEAD detached at fa0c4e1") then probably `git rev-parse --abbrev-rev` and `git rev-parse --short` should be used together. Unfortunately, while rev-parse can print multiple output in a single command, abbrev-rev and short cannot be used together. In such case, parsing `git status` might indeed be more suitable as it spawn only one executable instead of two.
Some porcelain commands do have --porcelain flag to ensure the stability of the interface, though (e.g. `git status --porcelain=v1`).