In the entry for "herbaceous": (hûr-bāshəs, ûr-)

According to the Usage Notes, both "an" and "a" are correct in written form (since both pronunciations are correct). If you're reading it out loud, "an herbacious" should be pronounced with a silent "h", and "a herbaceous" should be pronounced with an aspirated "h".

I think it would depend on pronunciation. Since "herbaceous" isn't a term you hear often, most people likely don't have a good instinct for how they "should" pronounce it. Seems plausible that the author concluded one pronunciation made more sense at one point during writing and the other pronunciation at a different point. Or maybe one was just a typo. ;-)