If it seems like he misses the drum fill cymbals on purpose, that's because he has the "optimum OverDrive path". It's the way to get the maximum points by using your OD at key times.
If you wonder how he got 100% but messed up his combo, that's from overhitting; he hit all the notes, but hit one too many. So he did hit 100% of the notes, he just broke his combo.