All or most anyway, not sure about the [chefs etc.][1], SE staff have special mod status. Presumably, this is part of the special way that they are handled.

Basically, that's not a regular user but a Stack Exchange employee so different rules apply.

As for the cases where they have <100 rep and have not posted, they have probably downvoted: 

[![3 down][2]](http://askubuntu.com/users/237/balpha?tab=summary)

Each downvote on an answer [costs you 1 rep][3], unless the downvoted post is later deleted.

Moreover, the negative reputation is not visible in reputation history of users to anybody but the user himself/herself.


  [1]: http://blog.stackoverflow.com/2013/09/summer-hiring-happened-so-fast/
  [2]: https://i.sstatic.net/kwUjX.png
  [3]: http://askubuntu.com/help/whats-reputation