It would be more correct to say that there are ALWAYS sixty-four signs in the Discworld zodiac BUT also that these are subject to change without notice. Stars immediately ahead of the Turtle's line of flight change their position only very gradually, as do the ones aft. The ones at right angles, however, may easily alter their relative positions in the lifetime of the average person, so there is a constant need for an updating of the zodiac. This is done for the Sto Plains by Unseen University, but communications with distant continents (who in any case have their own interpretations of the apparent shapes in the sky) are so slow that by the time any constellation is known Disc-wide it has already gone past. This does at least mean that astrology on the Disc is a dynamic thing and not a repository for some rather unimaginative mythology, but it does rather reduce the science to something on the lines of 'Look! There! No, THERE! Where I'm pointing! THERE! Doesn't that look like a crab to you? Oh, too late, you've missed it.'