They ask questions that use a broad variety of algorithms. I remember doing a contest that had like flow, centroid decomposition, fenwick tree, sqrt decomposition/segment tree, string algo, and dp all in the same contest I think. As much as I love CF, the number of different algos the questions involve is kind of limited. AtCoder probably has the narrowest range. Pretty much none of AtCoder's problems use algorithms that are outside the knowledge of a blue coder.