If you go bad in an exam but your assesment marks are good then you will have a good assesment mark with a lower exam mark (depending on how you went). Basically, what assesment mark you get depends on how everyone else goes as well. The better you all do, the better you all do.
They are good rankings, so if you do feel like you bombed an exam atleast you know your assesment mark will boost you up. It is half assesment, half exam so either way you think of it, it is your exam brings your assesment down, or your assesment brings your exam up - its the same thing.