As others have mentioned in this thread, many services will just not post a poor review. They have an incentive to make a sale, and a low star rating is going to turn a buyer away. Why push you away from the buy button when you're already on the page?
There's also the problem of incentivized reviews. I'm currently working on self-publishing a book. Standard practice is sending out "advanced reader copies" before you launch. The readers get a free book, you hopefully get some reviews to prime the pump on launch day. So far, somewhat reasonable.
But many of the platforms let you block reviewers. If you're a major publisher using something like NetGalley, you would be foolish not to filter out anyone who's ever handed out a bad review.
At best, the result is honest reviews from a non-representative sample. But it's even worse than that. The reviewers know the game; a bad review will take them off the free book gravy train. So they bias their reviews to not get cut off.
I don't know how you solve the review problem; the financial incentive for getting good reviews is huge. If you're willing to spend money, it is 100% possible to get non-representative or outright fake reviews that are indistinguishable from the real thing.