In this blog post http://blog.stackoverflow.com/2009/08/new-question-asker-features/ Jeff Atwood explicitly mentions that he considers 70% a good accept answer rate:
It is considered good manners to accept answers on your questions, eventually, but accepting answers is not required. I personally consider anything at 70% or over quite good, meaning you accept answers on 7 out of 10 questions that you ask. There are certainly cases where you don’t get an answer you like, or the question is inherently unanswerable.
Yet, his stackoverflow profile http://stackoverflow.com/users/1/jeff-atwood reveals that he has an accept answer rate of 100% (You might have to click on one of his questions to see the accept answer rate)
EDIT: In defense of myself. My rudimentary control of the English language actually allows me to understand the meaning of the word "over". However, that wasn't the focus of my question, rather: if Jeff states that not all questions are answerable, then why are all his questions answered? I'm not attacking him or throwing harsh accusations, I simply want to understand why Atwood is an exception to his own rule.
dmckee did give me the answer I was looking for.