So the question is why the file in this report with about the same size
and number of changes between minimized and prettified version result in
such strange numbers?

ljell is right, it seems to have to do with the non-ASCII characters.
In my sample.json.gz from above, I've just replaced every "e" with an
"Ê" (except in true/false literals).  When I prettify that, it aborts
early (fast) just after 449 early_abort_tests.