In the below "double," you can see that the jump consists of two mounds of dirt (hence the term double). The bike will take off from the first mound on the right, and jump to the left and land on the backside of the mound on the left-side of the frame. Double jumps take time to work up to, because a little mistake can cause the rider to come up short. On the flipside, a tabletop has no "gap" and no space to clear. Therefore, the risk of crashing if you come up short is minimized.