Here is a similar thread where we talked about it.
I think that Laston field will update to the current date the minute you log back in, so I don't think your theory works.
Note how the "last seen" pal on the index page will say "2 seconds" when you look at it yourself. Thus it would show you only posts posted in teh last few seconds.
I think it has to update that time field with every page view, because most people don't "log off" and thus it keeps updating until you stop viewing threads pages.