I think you're probably hitting on something that's true but often unsaid. Parents play a large role in facilitating their love for reading. You seem you're doing a great job. I assume many other parents are not.

As much as children having phones is an issue, I bet parent's having phones is an equally sized issue.

There's a selection bias of who reads and reviews books. For instance, Classics are read by a diverse group, including plenty who are reading a book in a style / genre/ etc. that they don't normally enjoy. This is going to cause a lower rating.

Its similar reason why sequels are often rated so high, especially in an area like epic fantasy. The only people who read the second book really like the first. This makes it easier for a sequel rating bump.

That said, I don't think I believe your rating system is some pro-tip, but rather a byproduct of your own preferences. There's plenty of people that can come to a completely different answer.