I've seen this pretty often and this time I can confirm it and rule it as a possible small bug or maybe a future improvement.
Consider this question. I asked it on 1/11/11 and I received an answer a few minutes later, which I then marked as accepted. on 1/12/11 at 11 PM EST (~36 hours later) I get an email from Stack Overflow letting me know that I have one answer for the question.
I know I did, that's why I marked it as accepted, hence it shouldn't have sent me the email as it doesn't make sense: there is nothing new to report. Why can't we be green and save a few bytes here and there.
Maybe the logic should be to only send an email if there is new activity since I last viewed the question. Or maybe use my last activity on that answer to know if I already read it: a comment to an answer, an up/down vote, marking an answer accepted, or the last time I viewed the question.
Just my 2 cents to make SO an the SE network even better than what it currently is.