Minion/CS Bug

So basically, I got over 500 farm in a game (for the icon) and I was in skype with a friend and after I hit 500, it looked like every minion kill brought me down. I ended at 480, which is a lot less than what I actually got. (It says 452 minion and 28 neutral, which isn't right). I was really proud of my CS ( with I always go for at least 400 farm) and It kinda pissed me off. I have a link to the match history showing the cs totals. Thanks! -Princess LoIi