I don't think anyone has really answered your question about why you encountered both in this article. "A herbacious," was only present inside a quote in the ar

I don't think anyone has really answered your question about why you encountered both in this article. "A herbacious," was only present inside a quote in the article. The author of the article seems to prefer using, "an," in their own writing. I don't know of any situational difference in pronunciation in play here, just regional or formal preferences as mentioned by other commenters.