Lydia's skills are in one-handed, heavy armor, archery, and block not sneak.Sven's skills are alchemy, enchanting, smithing, and archery.To this end, the official strategy guide states: Faendal is always going to do better with bows than with a greatsword, for example. These will improve with level, but nothing else will.
Followers are generally given a set of skills that will improve, regardless of what you equip them with.
