Since a crossbow bolt is not just an arrow, the bolts have different characteristics. They fall at the same rate, independent of the speed of flight. An arrow, in contrast depends on gaining lift in flight, and are made so the balance point is about 9% of the length forward of the center point if the arrow. This is called "point plaining".
Since the crossbow bolts are shot from the center of the crossbow, (an arrow doesn't have to be launched from the center point of a bow), the quarrel doesn't have to maintain the arrow spine in the same way.