Did you install Git through Homebrew? If so, then it also installed the completion bundled with Git, which is a simple port of its Bash completion, which then overrides Zsh's own much more sophisticated Git completion.
At least two issues and two PRs have been filed with Homebrew to make installing the completion shipped with the Git formula to be optional or remove it altogether, but its maintainer refuses to do so, because he feels using this Bash-style completion in Zsh is good enough for him. His argument is basically that since it ships with Git, it's the official completion and it should override Zsh's own (much better) Git completion. As to why this inferior completion is even maintained in the Git repo, when the Git manual itself recommends using Zsh's completion instead, I have no idea. I guess that's open source for you. ?????♂?
The only reliable way around this that I've found to work around this is to put the following line in one's .zshrc
file:
rm -f $HOMEBREW_PREFIX/share/zsh/site-functions/_git
This will cause Zsh's own completion function for Git to be used instead, which gives the much nicer output in the screenshot you posted.
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