I don't know this for a fact, but it's possible that the text run is logically considered to be Arial and the fallback could be handled as just a rendering step

I don't know this for a fact, but it's possible that the text run is logically considered to be Arial and the fallback could be handled as just a rendering step, rather than being encoded in the document. Doing it that way could allow the text to render on different versions of Arial, some of which do have a checkbox char, at the risk of the appearance and layout changing depending on which fonts are installed.