The odd part about what you observed is that new threads were only one day old. The setting for that is three days. After that time, all threads and posts are considered "read" even if you haven't been logged in.
So, are you sure it was 24 hours for all threads and posts and not possibly 72? The majority of threads will logically be showing as updated recently, but there should be some which weren't posted in for a few days.
The three day setting for marking posts read is a board wide parameter and is not changeable individually.