What J_menz said, basically. They're rough estimates. That said, if you're just now starting, you won't have to pay much attention to those for now, as you'll be doing basic level things. Once you get into higher level repertoire, such as Chopin mazurkas, nocturnes, etc, or Bach 2 part inventions, that's when you'll need to look at each of the systems, ask around a bit, sight read the score, and determine how hard you think it will be for YOU. What is easy for one person is hard for others, and vice versa.