2012 ATP World Tour Finals: results of round robin accurately mimic the world rankings?

We have Djokovic undefeated at 3-0: Number 1 in the world.

Next, we have Federer at 2-1 with 5 sets won/ 2 sets lost:  Number 2 in the world.

Murray is 2-1 with 5 sets won/ 3 sets lost:  Number 3 in the world.

Ferrer is also 2-1 with 4 sets won/ 3 sets lost: Number 4 in the world.

Here, the man who defies this logic is Del Potro, 2-1 with 5 sets won, 3 sets lost. Would you say Del Potro deserves to be considered more dangerous than Ferrer in this year end finals? Or would you say Ferrer is still the better man, more consistent and less up and down?