This best way to explain this I guess is with an example:
When I first registered, I typed my name as “Starbury”
Now, let’s say I log in the next day, and I type my name in as “starbury”.
When I start a thread (or I’m the latest poster to a thread) it will show my name on the thread list as “starbury”, but when you open the thread it will list it as “Starbury”. I assume the thread list page saves your name from the cookie/session id that you currently have, while the actual posts within the thread take your name from the database, where it was recorded when you initially registered.
What, you’ve never seen one of the several threads started by BeAr_nEnNo (or something)? I remember that he was rather fond of this quirk when it was first discovered.