Keep in mind that both sites are estimates - there's absolutely no way to determine which reviews are "fake" with 100% accuracy. The difference with ReviewMeta is that you can actually see the numbers behind our calculation and even open the hood and turn the knobs yourself.
I also agree that it should be very easy for Amazon to pick up on. I can't believe it's happening. We call this "Review Hijacking" and have a warning that appears at the top of our report if we detect it: https://reviewmeta.com/blog/amazon-review-hijacking/