The biggest factor in my own case is that in retrospect I have more respect for the difficult courses, and less respect for the easy courses, then I did at the time they actually took place. I think this is mostly because at the time I judged courses by whether they felt fair/unfair, and the easiest way for a professor to avoid students feeling they were judged unfairly is to just judge everyone in a broadly positive way. If you give some people Cs, inevitably some of them will feel they deserved higher (and sometimes they will even be right, since there is an error rate). While if you give everyone an A/B there's less cause for complaint, because many fewer students perceive getting a grade higher than they deserve as unfair.

A secondary factor is that when judging in retrospect I put a little less weight on personality and style than I did at the time. Professors with a "entertaining/engaging" style generally seemed good to me at the time, but in retrospect some of them were a bit shallow in the actual material (some were good, though). As an adult occasionally looking for online lectures, I now tend to specifically avoid the more "TED-style" lecturers, even though they are initially appealing, and find I get more out of a more straightforward, sober style.

Some of the really easy and "fun" courses in particular feel like a missed opportunity to me: we had a good time every week in class, I got an A, and I rated the course positively, but I've had to go back and self-study from a book because I had not actually learned much the first time around. If you asked me for the rating 5 years later, I would've revised it downwards!