There was discussion about low ratings on some apps and it was mentioned that a lot had to do with rate on delete. This has now been changed by Apple, much to the thrill of many developers.
This is indeed good news. It stands to reason if you are deleting an app, you at the very least did not find it useful. Soni am sure that most of the ratings done at that time would be negative. Perhaps now, overntime, the ratings will become more balanced.