[tsomer]

Party registrations were used to identify professors' political inclinations...the faculty ... leaned Democratic, especially among humanities professors. Using SAT scores as a proxy for the preparedness of students, the researchers were able to rule out patterns in which ... professors had better students.

So, reading between the lines here, no consideration was given for the field of study or the area of expertise of the professor.

[Redbob]

"So, reading between the lines here, no consideration was given for the field of study or the area of expertise of the professor."

By Jove, I think he's got it!

Where are the professors more likely to be the few remaining '60s hippies?
Liberal arts course, of course.

And in which field are the professors more likely to be Republicans?
Engineering.

Now, in which of these two fields are the correct answers on a quiz more purely objective, thus leading to a greater disparity in grades?
That's right: the Republican professors' engineering studies!

So: GOP-leaning professors are bound to give more grades on the extremes. QED