The astrological constellation boundaries and those drawn by the IAU are not the same thing. The astrologers use a map where each zodiacal constellation covers an equal amount of sky(30 degrees) no matter how large or small the constellation appears to the eye. So part of the current boundary of the Serpent Bearer crosses the path of the sun, but for astrological purposes it is counted as being part of the Scorpion.
The frog is right; there were originally 13 divisions to the zodiac, and there are still 13 months on the lunar callendar.