Most sites are that way, when I log in at bank, it will show last log in, doent KNOW what was done, just lists a time, so if you log off then back on it will list last visit as 2 minutes ago.
It says
Show new posts since last visit
so if you leave the site and come back it is considering this a new visit example you check in at 8 am before work, then log back in at noon, even though you didnt get to read everything at 8 am, that is the last time you VISITED