So, it's not about acting ability then...
Hmm.... It's about being a star. What is a "star," anyways? What qualities does a star have in order to be able to effectively rank those qualities?
Drawing power? As someone has already pointed out, Harrison Ford (#35) has drawn far far more to his movies than, say, Sidney Poitier (#20).
Intrigue of the People Magazine variety? It's probably safe to say that Steve McQueen (#31) and James Dean (#30) are probably slightly more interesting personalities than Tom Cruise (#3).
What is a star? What is being factored in? What are they measuring, exactly?
I have no idea... But I can tell you what they're NOT factoring in.
I posit, The Penn Factor.
Take it! Take it!