I have accepted an answer on every question that I posed, except for the question that got closed? Do closed questions count towards the accept rate?
The accept rate is updated every 24 hours (or so). Closed questions do not count when calculating the rate.
The mouse-over text says you've accepted 7 of 8 eligible questions. When I look at your account profile I see 11 questions, so three of them aren't eligible.
This appears to be the only question (with answers) that you haven't accepted an answer.
But, honestly, 88% is a great accept rate. Don't worry about it.
More information can be found in this FAQ on Accept Rate.