First, it looks like similar 10:1 rules show up in film, journalism, music, and photography! How cool is that?
Second, a common response is that even a single character change may show up in Git as an “inserted line” or “deleted line”, so when you see 100,000 lines were changed, it doesn’t mean that all the text in those lines was rewritten. This is true, but as I wrote above, there are also many types of changes missing from the data:
I don’t do a commit for every single line that I change. In fact, I may change a line 10 times, and commit only once.
This is actually even more pronounced for code. While doing a code-test cycle, I may change a few lines of code 50 times over, but only do one commit.
For my books, a lot of edit rounds and writing happened outside of Git (e.g., I wrote some of the chapters in Google Docs or Medium and O’Reilly does copyediting in a PDF).
My guess is that these two factors roughly cancel out. It won’t be exact, of course, and the actual ratio may be 8:1 or 12:1, but the order of magnitude is probably correct, and 10:1 is easier to remember.