If someone gives you negative rep. it subtracts from your reputation points. Bar are awarded at every 100 reputation points. After a while it starts being awarded after every 200, and after another while the number increases to 300.
You talking about the list of rep you got from the last 10 people? As you get rep it's constantly updated...So it may show rep you got from so and so gone, because the latest rep you received moved it off the page. If that' ain't it, I don't know what to tell you.