Intuitively this feels obvious. Content generated by the model will be shaped by its training, therefore when reading it back it will resonate with that same training and have a positive view as a result.

Human when preparing a CV: "Make my CV more professional"

LLM many days later presenting a report to HR: "This CV is really professional"

There's probably more to it than that of course.

But it justifies my personal policy of using a different LLM family for code review tasks than for code generation tasks. To avoid the "marking your own homework" problem.