Mind you that the difficulty bar is there mostly for new players.
So yeah, It's gonna be high for champions with a lot of skillshots, unusual mechanics/scaling (Ryze have a difficulty of 7 from what I recall, and that's because he scales with mana and works best with smart casts) etc.
For example - Ashe have difficulty rating of 4 and Caitlyn 6, while undoubtly Caitlyn should be the one easier to play - however, due to the fact that Caitlyn have her net which new players often misuse (using it on enemies while chasing etc) she got the higher rating.
So, if you are a level 30 players then most likely the difficulty bar means nothing to you at that point (or very little at least).
For both Ori and Syndra the high ratings are there because new players will not be able to keep track of their position/number and will also usually fail to understand how skills like her W works (it will take some time).
This is also the reason why I am laughing whenever someone says things like: "I rekt you with Lee and he is so hard to play" or "Garen muh skill. Play a real champion!". Though, I didn't see anything like that since Silver, which is good.