IB should have a better answer, but they probably won't be reading this for a while, so my thought is that it's probably down to cookies, and their expiration time.
The site is funded by advertising, and cookies are a necessary part of that process, so accepting cookies (and not deleting them after visiting) is a necessary evil.
Not sure why you don't want to tick the "remember me" box, as it would likely solve your problem - but again, that's down to cookies as well.
SD