When firing a crossbow with multishot, normally the arrows come out with one in the center and 2 about 15 degrees off it on each side. However, when firing nearly straight up or straight down, the arrows all come out at roughly the same angle, all aimed the same as the center arrow. There is likely some error in the trig the game is doing to figure out the angle, since it seems to just fire the 2 side arrows as if you are looking about 15 degrees right or left, which has a smaller, down to nonexistent effect, as a player looks closer to vertical.