Probably different versions installed. You can see version numbers on the bottom of the page. Lemmy.world hasn’t forked lemmy at all but instances could do that and be running something totally custom.
That’s possible because federated services use a standard protocol for communication called ActivityPub. This means they all talk the same “language” between instances separate from their frontend api or backend capabilities. So long as they adhere to the standard, different versions or entirely different software can be used for the backend and it doesn’t matter.