Ah – I see you have added the last visit date to the topic template.
You will have to tell me if this is not accurate. When a user logs out or is timed out of the system through inactivity, the last visited date is stamped at that precise time. This is then displayed the next time they visit. This certainly works fine here and we have no other reports of it not working. It has also been like this since about day 1 of SP back in 2006!
It’s a shame you have removed the post date and time from the posts as then it would better show up any anomalies. Same with removing your members popup profile in the case of the person who looks like they registered a day after their first post. I think this might actually be a possible event if the registration took place in that period where the user’s timezone was the other side of midnight to the server timezone. WP I believe, records the registration date according to the server setting. SP records the last visit date and post dates according to the users timezone.
I am curious why you removed the posting date/time from the posts though. Seems like useful information to display.