It is interesting, that changes from 8.0 to 8.1 is more substantial than from 8.1 to 9.0.

I don't say it to mean "they did less work," no, what his interesting to me is choice of major version bump instead of minor one.

The version number gaps are misleading.

Any change in major or minor number indicates a new release branch. And the convention since 5.0 has been to go 0 --> 1 --> 0 --> 1 ... for each new branch.