At Berkeley student evaluations of courses and instructors are still done on paper forms; we’re supposed to do them in class on a day when attendance is good. This is why I’m not doing them today – it’s Friday, and beautiful weather, and both of those always lower attendance. Although in practice, I tend to do the evaluations on a day when having a slightly shortened class makes sense, as opposed to introducing new ideas at the end of a normal-length class.
But let’s say an instructor is only interested in having the average of their evaluations be as large as possible. Wouldn’t it make sense to do the evaluations on a day when attendance is comparatively low? On a day with high attendance you’re likely to have the marginally interested students there, who would give lower evaluations. I would assume that the students who come every day like the class more.
(This is actually testable, if you could get everyone to fill out the evaluation. You could ask students “how often did you come to class?” and compare their self-reported attendance with their evaluations of the class.)
God plays dice 
Yes. I’ve thought of that. And there does seem to be a negative correlation between attendance and evaluations in my experience (higher proportion of attendees with lower course evaluation scores). But I haven’t run the numbers.