If I make any changes to a member in the admin control panel, such as change their avatar, that'll show up on the Recent Activity page. For example, "[Member] has a new avatar."
It's not that it shows there, it's that it shows it there if you do it via the admin control panel. If you go explicitly to http://xenforo.com/community/account/personal-details, for example, and change your avatar, then, of course, it's going to log that on the Recent Activity page. However, if you go into the admin control panel and change your avatar or someone else's avatar, it also logs it on the Recent Activity page.