The key phrase, I think, is "since last visit." If you're logged in forever, topics stay on unread for a long time, but you're bound to end up logged out and logging back in eventually. Everything that was there at the time of logging out gets wiped, and Unread Since Last Visit starts gathering new dust. New Replies keeps threads (only the ones you have posted in at least once) piling up in there for as long as you don't check back on them.