Plus, Ophiuchus/Serpentus is an actual sign of the Zodiac, making the total 13. People just tend to ignore and drop it from the lists, because the sun doesn't spend an entire calender month within it like it does the others.
Actually Ophiuschus/Serpentarius occupies more time within the period of what we know as Sagitarius, eating into over the first half of December. Dividing constellation into specific time frames is odd and actually rather vague due to the size differences of each constellations themselves, the axial precession of the earth and the change in positions of the sun and the constellations themselves. It used to be more accurate but we just haven't upgraded this 12 zodiac system ever since the times of the Greeks or Babylonians.
Ever since the 70s some people have even suggests the Cetus into being the 14th zodiac. Cetus being the whale constellation originated from the monster Perseus killed to save Andromeda in Greek myth, you know the thing that got replaced by Kraken in Clash of the Titans.