I'm not sure why have LastActivityDate and LastEditDate... do they not represent the same thing by definition?
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The last activity of a question includes not only changes to the question itself, but the addition of new answers as well as the editing of answers. You'll note the last activity timestamp, for any question which has had activity, to be posted on the right hand sidebar. This timestamp may also include the stealth bumping done by the Community User, which is otherwise untraceable.
However, I don't know if deletions/closures are counted under LastActivityDate, as they aren't tracked on the righthand side bar.