I originally noticed this on my iPhone using safari to view the desktop version. Today I noticed it too on my macbook pro. OSX 10.8.2. Please notice the time stamps on the activity in the screen shot.
This happened to me with the Mobile version thread...it doesn't show up on my desktop "recent activity" list, but it shows up just fine on the mobile website version.
Let's see if this thread gets moved to the top of my desktop activity.
Nope, it's still near the bottom of the desktop activity's first page.