The user I am talking about: balpha ♦
He has neither ever asked any question nor has he ever provided any answer on Ask Ubuntu. So, how does he have 100 rep. After association bonus, the rep count should be 101. Where did that unit rep go?
All or most anyway, not sure about the chefs etc., 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:
Each downvote on an answer costs you 1 rep, 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.