First, an 18 year old has less experience deciphering a thick accent than someone with years of experience in IT. I don't blink at accents that would have baffled me a decade ago.

Second, if there is cognitive load devoted to understanding the words, that is energy not devoted to understanding the concepts. Friends in those classes in college spent extra time outside of class with tutors and study groups than those with good teachers because they knew the tests didn't change.

Third, the lack of variance means the teachers don't make a difference in general. Perhaps there is a greater question to ask there.