Student learning is certainly not independent of the professor. For any given student, if they are more motivated to learn, they are almost certainly going to learn more, and professors can certainly influence that motivation level. That said, student evaluations have a lot of issues with them, and as @stradmom suggested, the comments are much more valuable than the faux-objective numerical scores on the survey. Students have a pretty substantial tendency to evaluate based on how difficult they found the class regardless of whether that difficulty was the result of poor teaching, poor study habits, or the material simply being inherently difficult. With the short-answer questions, it is much easier to remove the outliers who are evaluating based on what grade they got so that you can focus on the real, valuable feedback from students who put some thought into it.