Lemmy.world is run by Mastodon, while lemmy.ml is run by unapologetic socialists/communists.
One doesn’t really have a political focus while the other one has at least officially associated itself with such beliefs.
Thanks to the federated nature of lemmy, if you don’t like the way an instance is run you are free to go to another instance. If you want to be in an instance that will align with your beliefs, then you have that option as well. Isn’t it wonderful?