I'm sure they're taking in account more than actual work. Like:
Predicting - They probably based some of this off of what they think the model could do in the future. Also, if their look would give them longevity.
Social status - Anja who? Exactly. Fame is part of the modelling business, and most lists don't take it into account much. This one obviously does, or else Amber wouldn't be here. It does play a part in being a top face, and if people outside the fashion world know who you are then that's major IMO.
Newness - The ones who have been modelling for years aren't on here, leading me to believe that this mostly means the top 25 break out models of '09. They really should call it something like that, so people don't get confused when Lara, Anja, and Natasha aren't on there. There are models who have been modelling for a while, but really became big this year, such as Liu (who was known before, but she BLEW UP this year) or Jamie.