It's because often, FCing an easier difficulty can get you a higher score than getting, say, a B on a harder difficulty. All the B does then is not get you any points, and just brings down your accuracy, hence being pointless, because it brings down a statistic. At least, I'm pretty sure that's how most people like that think, and how I sometimes think. ;P
Go go pointless stats that people still care about for some reason! ...<_<; ...Including me sometimes. XP
and I recommend practicing with Spun Out. That way, you can 'balance' (as in, make your accuracy unaccurate) your statistics while chatting at the same time - the new way to do things! Then you go and get an S. Using Spun Out to practice also has other benefits, which I'm too lazy to mention. ex: Keeping your 8ness/9ness going.