You can go into your profile and click 'Find all posts by chibi_sunny' to see where you posted in - I had a look before I posted and I noticed you had posted in the What had you just ate? thread a couple of weeks ago, so that's part of it at least
You could check how many times you posted in a thread by hovering your cursor over the little folder image next to a thread title in a forum - it will tell you how many times you have posted in a thread. Another reason could be double posts - because of the server, people have made some double posts without realising it, a mod might have come along and merged them or deleted them. Sometimes it's just something you might not be aware of
