Not to threadjack, but in a similar vein I noticed today that when I clicked "Show new replies to your posts," a couple threads came up, but it was not ALL the threads I'd posted in which had new replies. I know because I happened to notice in the main forum that a thread I'd started had a new reply that I hadn't seen, but it hadn't come up in the "show new replies to your posts" list.
missmolly, your problem might indeed be related to the time difference you're seeing--from your profile you should be able to reset your local time. Sometimes it's hard to figure out which time to set it to, though, because I think the options are set up based on Greenwich Mean Time (GMT). I don't easily know how my time zone compares to GMT, so I just kind of guessed and then changed it again if I was wrong.