Well, why would the school give out tests to the same level unicorn fillies of various difficulties? Just askin'.
I'm still sticking to the opinion that Celestia was desperate, and after the Sunset Shimmer incident, set up a test to search for her the next wielder of the Crown of Magic. After she found Twilight, normal level, regular, reasonable tests were reinstated, as she no longer had a reason to give out impossible ones.