I'd like to see the demographic data on the Fox News viewers. If it skews older, less affluent, and less educated, that would explain a lot. It has been widely demonstrated (by Pew Research, in particular) that people falling into those demographics, especially less affluent and educated, are not as informed as others. They also vote at lower rates than the general population. I would guarantee you that the NPR listeners are more cosmopolitan, affluent, and educated; that is why they are better informed, not because they listen to NPR. Most of these surveys confuse correlation with causation.
Fairleigh Dickinson tries to downplay this aspect of the survey in their write-up, but these are really pretty small differences. Yeah, Fox viewers are the most uninformed/misinformed because they only get 1 of the questions right, but MSNBC viewers only get 1.2 questions right, which is the same as if they had watched nothing. About half of the NPR listeners also only get 1 question right. If NPR viewers were getting 4 or 5 of them correct, or even 3 correct, then the differences would be much more striking.
Sounds like the public generally speaking is badly uninformed.