This is not the scoreboard's fault, it's the challenges that are generally... funky.
If I keep track of my own kills/assists, it is often significantly different from what the scoreboard shows, both on the "personal" and "scoreboard" tabs. It's no wonder that people are reporting that challenges aren't keeping track, either. I find it hard to believe that the game programmers got some of the really hard stuff about designing a game right, where something that should be really basic programming, like counting kills, is so wrong.
