Not sure by what metric you compare the difficulty, but regardless of the hardness of the problem, IIRC, ICPC requires 100% correctness on test cases to score a problem (even failing one means you don't get the score,) but IOI would admit fractional scores (correct me if I am wrong.)
For fractional scores, it depends on problems. In short, there are two types of problems in IOI. One is traditional problems that requires 100% correctness, and the other is continuous scoring.
The prior can still results in score between 0 and 100, but this is because there are subtasks in the problem. For example, a graph become a tree or even just a linear sequence. Nonetheless, you still need to ensure your algorithm is correct on all testcases in that subtask in order to get the score of that task.
