It's a Google-specific legacy mistake. IIRC, they even do (used to do?) two-space indentation for Python.
> New projects should use that unmodified public Python Google style guide (4 space indent with methods_with_underscores). Historically, we adopted a style that was congruent with Google internal Python style guide (2 spaces with MethodsAsCamelCase).
https://chromium.googlesource.com/chromiumos/docs/+/HEAD/sty...