"But I might be an incredibly bad player and not abuse the mobility that costs my champion absolutely nothing or use it to do something stupid!"
That's not difficulty. Having actual limitations on what your champion can do, no get out of jail free cards, having to actually pay attention to positioning, and needing actual work to get in is difficulty.
Difficulty ratings are based on how many games on average, I think they probably used out of 100, it takes to be effective and efficient in games with a champion if you have never played them before.
Champions like Morgana are low and Champions like Singed are high. Something like Zed is rather difficult to play compared to Morgana because knowing when to go in and when to use your dashes and your skillshots is a lot harder than just using your Q, then placing your W and using your R and using your E if someone uses CC.
Cynical Giant was close.
Basically, Difficulty is generally how much practice it takes to understand a champion, combined arbitrarily with mechanics knowledge required to use them.
So, someone like taric would be fairly low. Low mechanics burden, and it only takes a few games to be competant with him. Compare that to, say, Shaco, and both the mechanics needed as well as the games to learn both shoot through the roof.
