In the Moderation Tools → Stats tab → Posts with notable activity → Most commented, recent heavy commenting activity doesn't show if the post is old, which means the tool only shows by Q/A creation (or last edit?) date, and not by commenting date (for the last 2 days for example).
I tried to search moderator-tools to see if it's been reported / asked about before, but I had no luck.
Screenshots in case comments are deleted, and for you to see what I'm seeing:
The post (What happens when lightning strikes an airplane?) was last edited in 2015 (edited Jan 6 '15 at 22:03). All the comments are 2 days old (and I'm viewing last 7 days for example).