The "All activity" page shows the questions that have had recent activity. It doesn't show the actual answer or comment text though.
So if a question/answer was just commented on, that question is listed at the top. You can click the "commented on by" link to jump to the actual comment, but the main link just shows the question.
The questions tab only shows questions by when they were asked so if you comment on a question asked 6 months ago, it will show under "all activity" but not "questions".