Where activity is shown, as in the 'active' tab (or there only) please distinguish the addition of a new post as an A from edits to posts.
Perhaps apply answer rather than modified in a case such as below:
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1 Answer
You can't see it because you don't have the privilege level to see deleted posts (2k rep for beta sites like this one, 10k rep for graduated sites), but that user has two answers on that question.
They posted one at 09:00:28 UTC today, then posted another, almost identical one at 15:31:37 UTC, and finally deleted the original answer at 15:31:54 UTC.
The post timeline is an excellent way to see the exact sequence of events, but you don't have enough rep yet to see the event for the deleted post. An excerpt:
It's the deletion of the first answer that causes the site to display "modified". If you had looked at the home page during the 17 seconds between their posting the second answer and deleting the first, you would have seen "answered" instead of "modified".
tl;dr this behavior is by design.