Seems fairly simple to me, here's what I understand of it:

There are 3 stats:

- Crit.Rate: A number, it corresponds to a certain amount of Crit depending on what level you are. A Crit.Rate of 1000 might be 20% at Lvl10 but only 12% at Lvl14.

(numbers only used as explanation)
- Crit: The actual percentage/chance you have on doing a Critical hit.

- Crit Dmg: The amount of extra damage you do when a critical hit happens.

This explains that Crit% adds directly to your chance of hitting critical.

While Crit.Rate% affects the Crit.Rate-number, which will affect your chance depending on your LVL.

I hope this explains it better, feel free to correct me if I'm wrong here!

edit: I think its the same with most other stats like defense and evasion, there's a raw number which corresponds to a % depending on the lvl you are..