I think it's because people download it either way (generally). If it's really good, it strikes them, so they are compelled to review. If it's really bad, they are so shocked by it's overall badness that they just have to give a review. Also, if a level has a really high or really low review, people are curious to what exactly it is, in case it's another case of new raters (no offense meant to those who are) rating something horribly off target. More middling rated levels don't quite fall prey to these.