A pc geek can correct me, but I think these are the main reasons.

*the IB software logs you off after about ten minutes of inactivity. The APC browsing history then unlinks you from the user name and links to the IP address you were on (I think). Since reading a thread page can easily take more than ten minutes we tend to get logged off and our browsing trail disassociated. This occurs even more when we change IPs frequently (travel).

*cookies may enter the equation as well, but browser behavior varies. Could be anything from no cookies, dispose of only when shutting down, dispose of when closing the browser, quite a few possibilities. Look into that.