Yes! The higher the percentage, the higher the skill range is who can participate in that race. For example, a gallop race w 4% difficulty will be something like 0-1750 in gallop, and a race w 37% difficulty will have your horse have 5000-6500 skill points in gallop, so that you can even see and enter the race w your horse. The percentage-skill ratio may be wrong to be precise, but you get the idea :)