Short answer: The joke is that astrology girls are crazy.

Context: If you look up your sign according to your date of birth (sun sign) and find that it doesn't fit your personality at all, well you just need to delve deeper, find out your rising sign and moon sign by having a professional (or algorithm) cross reference your time of birth with the fancy astrology chart. Still doesn't fit you? Well you probably just got the time wrong because it has to be to the exact minute. Perhaps it's that your mom doesn't remember correctly, or that many hospitals will fudge the times a little in your birth records. You must work extensively with a professional astrologer, who will assign your sign based on whichever one sounds the most like you. You need the 3 signs for your reading to really "work"