I can identify three scenarios.
The first is that the student's 85 is in the "middle" of the class, and there was nothing else memorable about the student's performance. There is no reason for the professor to "know" this student or write a recommendation.
The second is that the student got a 95 (or better) and was the top student in the class. Here, the professor is likely to know him or her, and could write a good recommendation. (I once got a 98 in a class, knew things that others didn't know, and was pleased to have the professor remember me 25 years later.)
The third situation is where the student got an 85, an "average" grade, with extenuating circumstances such as impaired vision or some other physical handicap, or temporary but severe psychological problems. Here, the professor is in a position to vouch for "extenuating circumstances" and the fact that the student is probably better than the 85 grade implies.