This is the most spot on comment. This is field- and technique- specific, but animals approved in a grant and animals that end up in the paper are, in my experi

This is the most spot on comment. This is field- and technique- specific, but animals approved in a grant and animals that end up in the paper are, in my experience, extremely unlikely to be the same number. Even in an experienced lab with well-oiled technical expertise, there's nearly always a fraction of animals that go to pilot experiments (i.e. didnt work), have some confound, or just didn't work. And by didn't work I mean didn't even get far enough to count as negative data, e.g. if you're trying something very technically demanding or new. Jumping to the conclusion that negative results aren't being reported (which is indeed its own big problem) from a difference in animal number suggests lack of understanding of the process, but perhaps formal accounting of research animals differs by country.