That’s probably because they could only identify the reviews that were honest enough to admit to being incentivized. The less honest incentivized reviews weren’t included because they can’t be easily identified, and are likely even more biased.
I think this is one of the more interesting issues confronting Amazon, which is that companies can test for favoritism and then concentrate their efforts on getting those reviewers to post. The reviews can even be individually unbiased, but if you can get an unrepresentative sample of unbiased reviewers to post, you can artificially inflate your ratings. So, again Amazon needs to ban all incentivized reviews, including filtering for positive bias.
