Think of the 4 "raids" as one raid split into 4 parts, with the trash removed (there might be a couple pulls here and there, like in A10/A11, but it seems they put those into the solo instances this time around) because no one likes trash mobs. This lets the raid work on only one specific boss if they want, without having to clear all the earlier bosses in the raid if they no longer need gear. Have all your loot from deltascape 1.0 for everyone in the raid? Just skip directly to 2.0, you've clearly proven you can handle 1.0 if everyone has every drop they need.

As has already been mentioned, we currently only have deltascape normal mode, with savage to be released in the next patch. Think of this as the story/casual mode, so that everyone who wants to can experience the story and get some small upgrades, and not feel left out. Deltascape Savage is going to be the "hard" version of the raid, with harder/more mechanics, etc.