They changed that system in the middle of last year. There is a ton of information out there if you are curious but here is one article:
https://www.theguardian.com/technology/2015/jun/22/amazon-ai-fake-reviews-star-ratings-astroturfing

They weigh reviews based on the age of the review, how helpful it is perceived, and a large number of other factors. Same thing I do except I removed sponsored reviews from the picture completely before creating a weighted average.

Here is an example:
https://www.amazon.com/gp/product/B016E1505S
Amazon rating: 4.1
Average of the 60 reviews: 4.3

Meanwhile I can't even put together enough quality reviews to rate the item because the sample of real and useful reviews is too small.

But the simple average of the remaining real and useful reviews is: 3.0
https://www.fakefilter.com/p/hotelspa-cool-shower-ultra-luxury-7-setting-7-inch-rainfall-shower-head