The reasons don't seem to be clearly stated, but I could see the very active social life described in the article affecting the quantity (not quality) relative to other interns. The paraphrasing of her manager seems to lead in that direction.
Would you hire her?
Women tend to put much more emphasis on these social parts, and its likely even more pronounced considering that this part is probably the only thing that she feels good about.. even now after the final experience.
Also, you're surrounded by a bunch of other interns your age, and the whole program is structured around socializing with them. Years ago, when I interned there, pretty much every weekend there was a different company-sanctioned intern event (hike Mt. Rainier, dinner, movie, go to Woodland Park zoo, concert, etc.).