There are several variables in the shape of each horse's mouth; such as
height of palate (some horses have very low palates); height, width, length of bars;
shape of bars (thick, thin, rounded, pointed, squared); distance
between bars; thick or thin tongue as well as wide or narrow; wolf teeth, etc.
Prior damage to any of the landmarks within the mouth, including lips, can affect how a
horse perceives a bit, and whether or not it is comfortable for him.
For the horses with low palates, such as Icelandic Horses, single jointed snaffles may
bother them. Some prefer double-jointed snaffles, some prefer mullen-mouth bits.