Different types of problems. In Atcoder, you often need that right way of looking at the problem rather than going through common approaches and figuring out what can be applied and how.

Take it this way: solving (upsolving, mostly, given how few AGCs there are) Atcoder problems gives you a new skillset.

off the topic, your coding style is very similar to tourist xD;

off topic: You should count how many "M"s ur rating graph has