Basically everybody who applies for a cadetship will get one, it just depends on which firm you go to ie if youre better than someone else youre more likely to get a cadetship at one of the better, well renowned firms.
I know plenty of people who got cadetships and there isnt anything special about them at all (not a great atar and hardly any extra curriculars) but I also know of people who got cadetships and really deserved it. The difference is the ones who deserved it are at the better firms.