The difficulty value is determined by range, height, cover (such as a hill), and movement (such as a running horse). The highest difficulty is 14.4 and the lowest is 1.0. The difficulty is decreased by Horse Archery and Power Draw, which both enable you to do harder shots.
Head shots deal much more damage to the target you hit, and increase your shot difficulty even at minimal distance. Therefore, at high enough proficiency and skill levels, head shots almost guarantee that any target will be dead with one shot.
Target density has no effect on shot difficulty. When shooting at a large and compact group of soldiers from a long distance, it is possible to easily get "difficult" shots, allowing for quick ranged weapon leveling.