Mitchell,
Dana Hocking said "Each of the effects is rated for difficulty on a scale of 1 to 10. These are of course subjective ratings, but they should give you a general feel for the difficulty of the effect / flourish you're interested in."
I really don't ever care what the skill level is for anything...
if I really want to learn whatever it is I want to learn, I will learn it
Personally, I don't like this rating thing. If the rating is high, I feel intimidated to learn the sleight and I might not feel as confident performing it. On the other hand, if the rating is low, then I'll keep telling myself it's easy to do and probably spend less time practicing it....Well, thats just me