From Nate Silver's "The signal and the noise:"

The amount of information was increasing much more rapidly than our understanding of what to do with it, or our ability to differentiate the useful information from the mistruths.

Why is there a comma before or? Isn't the second clause dependent, since it refers to the increasing amount of information (so it's not self-contained)? If it is indeed independent, how exactly do you tell when a clause is dependent?